From: Peter Zijlstra <peterz@infradead.org>
Add a Kconfig option for debug builds which logs a warning when an
instrumented atomic operation takes place at some location that isn't
a long word boundary. Some platforms don't trap for this.
Link: https://lore.kernel.org/lkml/20250901093600.GF4067720@noisy.programming.kicks-ass.net/
---
This patch differs slightly from Peter's code which checked for natural
alignment.
---
include/linux/instrumented.h | 4 ++++
lib/Kconfig.debug | 10 ++++++++++
2 files changed, 14 insertions(+)
diff --git a/include/linux/instrumented.h b/include/linux/instrumented.h
index 711a1f0d1a73..55f5685971a1 100644
--- a/include/linux/instrumented.h
+++ b/include/linux/instrumented.h
@@ -7,6 +7,7 @@
#ifndef _LINUX_INSTRUMENTED_H
#define _LINUX_INSTRUMENTED_H
+#include <linux/bug.h>
#include <linux/compiler.h>
#include <linux/kasan-checks.h>
#include <linux/kcsan-checks.h>
@@ -67,6 +68,7 @@ static __always_inline void instrument_atomic_read(const volatile void *v, size_
{
kasan_check_read(v, size);
kcsan_check_atomic_read(v, size);
+ WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
}
/**
@@ -81,6 +83,7 @@ static __always_inline void instrument_atomic_write(const volatile void *v, size
{
kasan_check_write(v, size);
kcsan_check_atomic_write(v, size);
+ WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
}
/**
@@ -95,6 +98,7 @@ static __always_inline void instrument_atomic_read_write(const volatile void *v,
{
kasan_check_write(v, size);
kcsan_check_atomic_read_write(v, size);
+ WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
}
/**
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index ebe33181b6e6..d82626b7d6be 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1363,6 +1363,16 @@ config DEBUG_PREEMPT
depending on workload as it triggers debugging routines for each
this_cpu operation. It should only be used for debugging purposes.
+config DEBUG_ATOMIC
+ bool "Debug atomic variables"
+ depends on DEBUG_KERNEL
+ help
+ If you say Y here then the kernel will add a runtime alignment check
+ to atomic accesses. Useful for architectures that do not have trap on
+ mis-aligned access.
+
+ This option has potentially significant overhead.
+
menu "Lock Debugging (spinlocks, mutexes, etc...)"
config LOCK_DEBUGGING_SUPPORT
--
2.49.1On Sun, Sep 14, 2025 at 10:45:29AM +1000, Finn Thain wrote:
> From: Peter Zijlstra <peterz@infradead.org>
>
> Add a Kconfig option for debug builds which logs a warning when an
> instrumented atomic operation takes place at some location that isn't
> a long word boundary. Some platforms don't trap for this.
>
> Link: https://lore.kernel.org/lkml/20250901093600.GF4067720@noisy.programming.kicks-ass.net/
> ---
> This patch differs slightly from Peter's code which checked for natural
> alignment.
> ---
> include/linux/instrumented.h | 4 ++++
> lib/Kconfig.debug | 10 ++++++++++
> 2 files changed, 14 insertions(+)
>
> diff --git a/include/linux/instrumented.h b/include/linux/instrumented.h
> index 711a1f0d1a73..55f5685971a1 100644
> --- a/include/linux/instrumented.h
> +++ b/include/linux/instrumented.h
> @@ -7,6 +7,7 @@
> #ifndef _LINUX_INSTRUMENTED_H
> #define _LINUX_INSTRUMENTED_H
>
> +#include <linux/bug.h>
> #include <linux/compiler.h>
> #include <linux/kasan-checks.h>
> #include <linux/kcsan-checks.h>
> @@ -67,6 +68,7 @@ static __always_inline void instrument_atomic_read(const volatile void *v, size_
> {
> kasan_check_read(v, size);
> kcsan_check_atomic_read(v, size);
> + WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
> }
>
> /**
> @@ -81,6 +83,7 @@ static __always_inline void instrument_atomic_write(const volatile void *v, size
> {
> kasan_check_write(v, size);
> kcsan_check_atomic_write(v, size);
> + WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
> }
>
> /**
> @@ -95,6 +98,7 @@ static __always_inline void instrument_atomic_read_write(const volatile void *v,
> {
> kasan_check_write(v, size);
> kcsan_check_atomic_read_write(v, size);
> + WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
> }
Right, so why aren't we trusting the size argument? And instead
mandating a possibly larger alignment?
Note how things like test_and_set_bit() will use sizeof(long), while
atomic_set() will use sizeof(*v), which, on LP64 architectures are very
much not the same.
The same with atomic_*() vs atomic_long_*() / atomic64_*(), they will
have different alignment requirements.
And then there is cmpxchg(), that can be u8 u16, u32 and u64 depending
on the user. And then there is cmpxchg128().
I really don't see how using long here is correct.
On Mon, 15 Sep 2025, Peter Zijlstra wrote:
> On Sun, Sep 14, 2025 at 10:45:29AM +1000, Finn Thain wrote:
> > From: Peter Zijlstra <peterz@infradead.org>
> >
> > Add a Kconfig option for debug builds which logs a warning when an
> > instrumented atomic operation takes place at some location that isn't
> > a long word boundary. Some platforms don't trap for this.
> >
> > Link: https://lore.kernel.org/lkml/20250901093600.GF4067720@noisy.programming.kicks-ass.net/
> > ---
> > This patch differs slightly from Peter's code which checked for natural
> > alignment.
> > ---
> > include/linux/instrumented.h | 4 ++++
> > lib/Kconfig.debug | 10 ++++++++++
> > 2 files changed, 14 insertions(+)
> >
> > diff --git a/include/linux/instrumented.h b/include/linux/instrumented.h
> > index 711a1f0d1a73..55f5685971a1 100644
> > --- a/include/linux/instrumented.h
> > +++ b/include/linux/instrumented.h
> > @@ -7,6 +7,7 @@
> > #ifndef _LINUX_INSTRUMENTED_H
> > #define _LINUX_INSTRUMENTED_H
> >
> > +#include <linux/bug.h>
> > #include <linux/compiler.h>
> > #include <linux/kasan-checks.h>
> > #include <linux/kcsan-checks.h>
> > @@ -67,6 +68,7 @@ static __always_inline void instrument_atomic_read(const volatile void *v, size_
> > {
> > kasan_check_read(v, size);
> > kcsan_check_atomic_read(v, size);
> > + WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
> > }
> >
> > /**
> > @@ -81,6 +83,7 @@ static __always_inline void instrument_atomic_write(const volatile void *v, size
> > {
> > kasan_check_write(v, size);
> > kcsan_check_atomic_write(v, size);
> > + WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
> > }
> >
> > /**
> > @@ -95,6 +98,7 @@ static __always_inline void instrument_atomic_read_write(const volatile void *v,
> > {
> > kasan_check_write(v, size);
> > kcsan_check_atomic_read_write(v, size);
> > + WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
> > }
>
> Right, so why aren't we trusting the size argument? And instead
> mandating a possibly larger alignment?
>
It wasn't supposed to mandate a larger alignment in practice. I considered
doing something like (unsigned long)v & (size - 1) & (sizeof(long) - 1)
but decided that the extra overhead probably wouldn't be worthwhile, if in
practice, no-one is doing atomic ops on shorts or chars. I will revisit
this.
When you do atomic operations on atomic_t or atomic64_t, (sizeof(long)
- 1) probably doesn't make much sense. But atomic operations get used on
scalar types (aside from atomic_t and atomic64_t) that don't have natural
alignment. Please refer to the other thread about this:
https://lore.kernel.org/all/ed1e0896-fd85-5101-e136-e4a5a37ca5ff@linux-m68k.org/
On Mon, Sep 15, 2025 at 07:38:52PM +1000, Finn Thain wrote:
>
> On Mon, 15 Sep 2025, Peter Zijlstra wrote:
>
> > On Sun, Sep 14, 2025 at 10:45:29AM +1000, Finn Thain wrote:
> > > From: Peter Zijlstra <peterz@infradead.org>
> > >
> > > Add a Kconfig option for debug builds which logs a warning when an
> > > instrumented atomic operation takes place at some location that isn't
> > > a long word boundary. Some platforms don't trap for this.
> > >
> > > Link: https://lore.kernel.org/lkml/20250901093600.GF4067720@noisy.programming.kicks-ass.net/
> > > ---
> > > This patch differs slightly from Peter's code which checked for natural
> > > alignment.
> > > ---
> > > include/linux/instrumented.h | 4 ++++
> > > lib/Kconfig.debug | 10 ++++++++++
> > > 2 files changed, 14 insertions(+)
> > >
> > > diff --git a/include/linux/instrumented.h b/include/linux/instrumented.h
> > > index 711a1f0d1a73..55f5685971a1 100644
> > > --- a/include/linux/instrumented.h
> > > +++ b/include/linux/instrumented.h
> > > @@ -7,6 +7,7 @@
> > > #ifndef _LINUX_INSTRUMENTED_H
> > > #define _LINUX_INSTRUMENTED_H
> > >
> > > +#include <linux/bug.h>
> > > #include <linux/compiler.h>
> > > #include <linux/kasan-checks.h>
> > > #include <linux/kcsan-checks.h>
> > > @@ -67,6 +68,7 @@ static __always_inline void instrument_atomic_read(const volatile void *v, size_
> > > {
> > > kasan_check_read(v, size);
> > > kcsan_check_atomic_read(v, size);
> > > + WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
> > > }
> > >
> > > /**
> > > @@ -81,6 +83,7 @@ static __always_inline void instrument_atomic_write(const volatile void *v, size
> > > {
> > > kasan_check_write(v, size);
> > > kcsan_check_atomic_write(v, size);
> > > + WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
> > > }
> > >
> > > /**
> > > @@ -95,6 +98,7 @@ static __always_inline void instrument_atomic_read_write(const volatile void *v,
> > > {
> > > kasan_check_write(v, size);
> > > kcsan_check_atomic_read_write(v, size);
> > > + WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
> > > }
> >
> > Right, so why aren't we trusting the size argument? And instead
> > mandating a possibly larger alignment?
> >
>
> It wasn't supposed to mandate a larger alignment in practice. I considered
> doing something like (unsigned long)v & (size - 1) & (sizeof(long) - 1)
> but decided that the extra overhead probably wouldn't be worthwhile, if in
> practice, no-one is doing atomic ops on shorts or chars. I will revisit
> this.
atomic_t is aligned at 4 bytes, you're now mandating it is aligned at 8
bytes (on LP64), this cannot be right.
kernel/locking/qspinlock.c:xchg_tail() does xchg_relaxed(&lock->tail,
...) which is u16. Again, you cannot mandate 8 bytes here.
> When you do atomic operations on atomic_t or atomic64_t, (sizeof(long)
> - 1) probably doesn't make much sense. But atomic operations get used on
> scalar types (aside from atomic_t and atomic64_t) that don't have natural
> alignment. Please refer to the other thread about this:
> https://lore.kernel.org/all/ed1e0896-fd85-5101-e136-e4a5a37ca5ff@linux-m68k.org/
Perhaps set ARCH_SLAB_MINALIGN ?
On Mon, 15 Sep 2025, Peter Zijlstra wrote:
> On Mon, Sep 15, 2025 at 07:38:52PM +1000, Finn Thain wrote:
> >
> > On Mon, 15 Sep 2025, Peter Zijlstra wrote:
> >
> > > On Sun, Sep 14, 2025 at 10:45:29AM +1000, Finn Thain wrote:
> > > > From: Peter Zijlstra <peterz@infradead.org>
> > > >
> > > > Add a Kconfig option for debug builds which logs a warning when an
> > > > instrumented atomic operation takes place at some location that isn't
> > > > a long word boundary. Some platforms don't trap for this.
> > > >
> > > > Link: https://lore.kernel.org/lkml/20250901093600.GF4067720@noisy.programming.kicks-ass.net/
> > > > ---
> > > > This patch differs slightly from Peter's code which checked for natural
> > > > alignment.
> > > > ---
> > > > include/linux/instrumented.h | 4 ++++
> > > > lib/Kconfig.debug | 10 ++++++++++
> > > > 2 files changed, 14 insertions(+)
> > > >
> > > > diff --git a/include/linux/instrumented.h b/include/linux/instrumented.h
> > > > index 711a1f0d1a73..55f5685971a1 100644
> > > > --- a/include/linux/instrumented.h
> > > > +++ b/include/linux/instrumented.h
> > > > @@ -7,6 +7,7 @@
> > > > #ifndef _LINUX_INSTRUMENTED_H
> > > > #define _LINUX_INSTRUMENTED_H
> > > >
> > > > +#include <linux/bug.h>
> > > > #include <linux/compiler.h>
> > > > #include <linux/kasan-checks.h>
> > > > #include <linux/kcsan-checks.h>
> > > > @@ -67,6 +68,7 @@ static __always_inline void instrument_atomic_read(const volatile void *v, size_
> > > > {
> > > > kasan_check_read(v, size);
> > > > kcsan_check_atomic_read(v, size);
> > > > + WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
> > > > }
> > > >
> > > > /**
> > > > @@ -81,6 +83,7 @@ static __always_inline void instrument_atomic_write(const volatile void *v, size
> > > > {
> > > > kasan_check_write(v, size);
> > > > kcsan_check_atomic_write(v, size);
> > > > + WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
> > > > }
> > > >
> > > > /**
> > > > @@ -95,6 +98,7 @@ static __always_inline void instrument_atomic_read_write(const volatile void *v,
> > > > {
> > > > kasan_check_write(v, size);
> > > > kcsan_check_atomic_read_write(v, size);
> > > > + WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
> > > > }
> > >
> > > Right, so why aren't we trusting the size argument? And instead
> > > mandating a possibly larger alignment?
> > >
> >
> > It wasn't supposed to mandate a larger alignment in practice. I considered
> > doing something like (unsigned long)v & (size - 1) & (sizeof(long) - 1)
> > but decided that the extra overhead probably wouldn't be worthwhile, if in
> > practice, no-one is doing atomic ops on shorts or chars. I will revisit
> > this.
>
> atomic_t is aligned at 4 bytes, you're now mandating it is aligned at 8
> bytes (on LP64), this cannot be right.
>
> kernel/locking/qspinlock.c:xchg_tail() does xchg_relaxed(&lock->tail,
> ...) which is u16. Again, you cannot mandate 8 bytes here.
>
OK. I will change it back to your code (i.e. mandate natural alignment).
> > When you do atomic operations on atomic_t or atomic64_t, (sizeof(long)
> > - 1) probably doesn't make much sense. But atomic operations get used on
> > scalar types (aside from atomic_t and atomic64_t) that don't have natural
> > alignment. Please refer to the other thread about this:
> > https://lore.kernel.org/all/ed1e0896-fd85-5101-e136-e4a5a37ca5ff@linux-m68k.org/
>
> Perhaps set ARCH_SLAB_MINALIGN ?
>
That's not going to help much. The 850 byte offset of task_works into
struct task_struct and the 418 byte offset of exit_state in struct
task_struct are already misaligned.
But that's all moot, if you intended that CONFIG_DEBUG_ATOMIC should
complain about any deviation from natural alignment. I still don't have
any performance measurements but I'm willing to assume there's a penalty
for such deviation.
On Mon, Sep 15, 2025, at 12:37, Finn Thain wrote:
> On Mon, 15 Sep 2025, Peter Zijlstra wrote:
>>
>> > When you do atomic operations on atomic_t or atomic64_t, (sizeof(long)
>> > - 1) probably doesn't make much sense. But atomic operations get used on
>> > scalar types (aside from atomic_t and atomic64_t) that don't have natural
>> > alignment. Please refer to the other thread about this:
>> > https://lore.kernel.org/all/ed1e0896-fd85-5101-e136-e4a5a37ca5ff@linux-m68k.org/
>>
>> Perhaps set ARCH_SLAB_MINALIGN ?
>>
>
> That's not going to help much. The 850 byte offset of task_works into
> struct task_struct and the 418 byte offset of exit_state in struct
> task_struct are already misaligned.
Has there been any progress on building m68k kernels with -mint-align?
IIRC there are only a small number of uapi structures that need
__packed annotations to maintain the existing syscall ABI.
Arnd
On Mon, 15 Sep 2025, Arnd Bergmann wrote: > On Mon, Sep 15, 2025, at 12:37, Finn Thain wrote: > > On Mon, 15 Sep 2025, Peter Zijlstra wrote: > >> > >> > When you do atomic operations on atomic_t or atomic64_t, (sizeof(long) > >> > - 1) probably doesn't make much sense. But atomic operations get used on > >> > scalar types (aside from atomic_t and atomic64_t) that don't have natural > >> > alignment. Please refer to the other thread about this: > >> > https://lore.kernel.org/all/ed1e0896-fd85-5101-e136-e4a5a37ca5ff@linux-m68k.org/ > >> > >> Perhaps set ARCH_SLAB_MINALIGN ? > >> > > > > That's not going to help much. The 850 byte offset of task_works into > > struct task_struct and the 418 byte offset of exit_state in struct > > task_struct are already misaligned. > > Has there been any progress on building m68k kernels with -mint-align? Not that I know of. > IIRC there are only a small number of uapi structures that need > __packed annotations to maintain the existing syscall ABI. > Packing uapi structures (and adopting -malign-int) sounds easier than the alternative, which might be to align certain internal kernel struct members, on a case-by-case basis, where doing so could be shown to improve performance on some architecture or other (while keeping -mno-align-int). Well, it's easy to find all the structs that belong to the uapi, but it's not easy to find all the internal kernel structs that describe MMIO registers. For -malign-int, both kinds of structs are a problem. If better performance is to be had, my guess is that aligning atomic_t will get 80% of it (just an appeal to the Pareto principle, FWIW...)
On Tue, Sep 16, 2025, at 02:16, Finn Thain wrote:
> On Mon, 15 Sep 2025, Arnd Bergmann wrote:
>
>> IIRC there are only a small number of uapi structures that need
>> __packed annotations to maintain the existing syscall ABI.
>>
>
> Packing uapi structures (and adopting -malign-int) sounds easier than the
> alternative, which might be to align certain internal kernel struct
> members, on a case-by-case basis, where doing so could be shown to improve
> performance on some architecture or other (while keeping -mno-align-int).
>
> Well, it's easy to find all the structs that belong to the uapi, but it's
> not easy to find all the internal kernel structs that describe MMIO
> registers. For -malign-int, both kinds of structs are a problem.
Right, especially since those structure definitions are more
likely to be used on older drivers, there are probably some that
do happen on m68k. On the other hand, any driver that is portable
to non-m68k targets won't have this problem.
I tried a trivial m68k defconfig build with "-Wpadded -malign-int"
and found 3021 instances of structure padding, compared to 2271
without -malign-int. That is still something one could very reasonably
go through with 'vim -q output', as almost all of them are obviously
kernel-internal structures and not problematic.
A quick manual scan only shows a number of uapi structures but very
few drivers. There are a few hardware structures that are internally
packed but in a structure that has gets an extra two bytes of harmless
padding at the end (struct CUSTOM, struct amiga_hw_present,
struct atari_hw_present, struct TT_DMA, struct ppc_regs). The only
ones I could find that actually seemed suspicious are:
arch/m68k/include/asm/atarihw.h:426:10: warning: padding struct to align 'dst_address' [-Wpadded]
arch/m68k/include/asm/openprom.h:76:13: warning: padding struct to align 'boot_dev_ctrl' [-Wpadded]
drivers/net/ethernet/i825xx/82596.c:250:1: warning: padding struct size to alignment boundary
I'm sure there are a few more, but probably not a lot more.
See https://pastebin.com/Z2bjnD0G for the full list. I've also
tried annotating the ones that show up in defconfig, see diffstat
below.
Unfortunately I found no way of annotating a struct a whole to use
the traditional padding rules: "#pragma pack(push, 2)" lowers the
alignment of all struct members to two bytes, including those
with an explicit alignment like __aligned_u64. A struct-wide
__attribute__((packed, aligned(2))) seems even worse, as it
makes all members inside of the struck tightly packed and only
aligns the struct itself.
This means all misaligned members have to be individually marked
as __packed, unless someone comes up with another type of
annotation.
> If better performance is to be had, my guess is that aligning atomic_t
> will get 80% of it (just an appeal to the Pareto principle, FWIW...)
arch/m68k selects CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS for anything
other than Dragonball, so I would not expect much performance difference
at all, unless CASL on unaligned data ends up causing alignment traps
as it does on most architectures.
Arnd
arch/m68k/atari/atakeyb.c | 2 +-
arch/m68k/include/asm/amigahw.h | 4 +--
arch/m68k/include/asm/atarihw.h | 6 ++--
arch/m68k/include/asm/mvme147hw.h | 2 +-
arch/m68k/include/asm/openprom.h | 2 +-
arch/m68k/include/uapi/asm/ptrace.h | 2 +-
arch/m68k/include/uapi/asm/sigcontext.h | 2 +-
arch/m68k/include/uapi/asm/stat.h | 4 +--
drivers/net/ethernet/i825xx/82596.c | 4 +--
include/uapi/asm-generic/termios.h | 2 +-
include/uapi/linux/acct.h | 2 +-
include/uapi/linux/ax25.h | 6 ++--
include/uapi/linux/blktrace_api.h | 2 +-
include/uapi/linux/btrfs.h | 2 +-
include/uapi/linux/cdrom.h | 36 +++++++++++-----------
include/uapi/linux/dlm.h | 2 +-
include/uapi/linux/dqblk_xfs.h | 2 +-
include/uapi/linux/ethtool.h | 14 ++++-----
include/uapi/linux/fb.h | 6 ++--
include/uapi/linux/fd.h | 6 ++--
include/uapi/linux/filter.h | 2 +-
include/uapi/linux/hdlc/ioctl.h | 6 ++--
include/uapi/linux/hiddev.h | 2 +-
include/uapi/linux/i2c-dev.h | 2 +-
include/uapi/linux/i2c.h | 2 +-
include/uapi/linux/if.h | 2 +-
include/uapi/linux/if_arcnet.h | 4 +--
include/uapi/linux/if_bonding.h | 2 +-
include/uapi/linux/if_bridge.h | 14 ++++-----
include/uapi/linux/if_link.h | 2 +-
include/uapi/linux/if_plip.h | 2 +-
include/uapi/linux/if_pppox.h | 2 +-
include/uapi/linux/if_vlan.h | 2 +-
include/uapi/linux/inet_diag.h | 2 +-
include/uapi/linux/input.h | 4 +--
include/uapi/linux/ip6_tunnel.h | 4 +--
include/uapi/linux/kd.h | 2 +-
include/uapi/linux/llc.h | 2 +-
include/uapi/linux/loop.h | 2 +-
include/uapi/linux/mctp.h | 4 +--
include/uapi/linux/mptcp.h | 2 +-
include/uapi/linux/mroute.h | 4 +--
include/uapi/linux/mroute6.h | 6 ++--
include/uapi/linux/msdos_fs.h | 2 +-
include/uapi/linux/msg.h | 6 ++--
include/uapi/linux/mtio.h | 2 +-
include/uapi/linux/netfilter/ipset/ip_set.h | 4 +--
include/uapi/linux/netfilter/nf_nat.h | 2 +-
include/uapi/linux/netfilter/x_tables.h | 8 ++---
include/uapi/linux/netfilter/xt_HMARK.h | 2 +-
include/uapi/linux/netfilter/xt_TPROXY.h | 4 +--
include/uapi/linux/netfilter/xt_connbytes.h | 2 +-
include/uapi/linux/netfilter/xt_connmark.h | 6 ++--
include/uapi/linux/netfilter/xt_conntrack.h | 4 +--
include/uapi/linux/netfilter/xt_esp.h | 2 +-
include/uapi/linux/netfilter/xt_hashlimit.h | 6 ++--
include/uapi/linux/netfilter/xt_helper.h | 2 +-
include/uapi/linux/netfilter/xt_ipcomp.h | 2 +-
include/uapi/linux/netfilter/xt_iprange.h | 2 +-
include/uapi/linux/netfilter/xt_l2tp.h | 2 +-
include/uapi/linux/netfilter/xt_mac.h | 2 +-
include/uapi/linux/netfilter/xt_mark.h | 2 +-
include/uapi/linux/netfilter/xt_owner.h | 2 +-
include/uapi/linux/netfilter/xt_realm.h | 2 +-
include/uapi/linux/netfilter/xt_recent.h | 4 +--
include/uapi/linux/netfilter/xt_set.h | 4 +--
include/uapi/linux/netfilter/xt_statistic.h | 2 +-
include/uapi/linux/netfilter/xt_tcpmss.h | 2 +-
include/uapi/linux/netfilter/xt_tcpudp.h | 2 +-
include/uapi/linux/netfilter/xt_time.h | 2 +-
include/uapi/linux/netfilter/xt_u32.h | 6 ++--
include/uapi/linux/netfilter_arp/arp_tables.h | 2 +-
include/uapi/linux/netfilter_arp/arpt_mangle.h | 2 +-
include/uapi/linux/netfilter_bridge/ebt_802_3.h | 2 +-
include/uapi/linux/netfilter_bridge/ebt_arp.h | 2 +-
include/uapi/linux/netfilter_bridge/ebt_arpreply.h | 2 +-
include/uapi/linux/netfilter_bridge/ebt_mark_m.h | 2 +-
include/uapi/linux/netfilter_bridge/ebt_nat.h | 2 +-
include/uapi/linux/netfilter_bridge/ebt_stp.h | 4 +--
include/uapi/linux/netfilter_bridge/ebt_vlan.h | 2 +-
include/uapi/linux/netfilter_ipv4/ipt_ah.h | 2 +-
include/uapi/linux/netfilter_ipv6/ip6_tables.h | 2 +-
include/uapi/linux/netfilter_ipv6/ip6t_ah.h | 2 +-
include/uapi/linux/netfilter_ipv6/ip6t_frag.h | 2 +-
include/uapi/linux/netfilter_ipv6/ip6t_opts.h | 2 +-
include/uapi/linux/netfilter_ipv6/ip6t_rt.h | 2 +-
include/uapi/linux/netfilter_ipv6/ip6t_srh.h | 2 +-
include/uapi/linux/netrom.h | 2 +-
include/uapi/linux/nfs_idmap.h | 2 +-
include/uapi/linux/nfs_mount.h | 2 +-
include/uapi/linux/pkt_cls.h | 2 +-
include/uapi/linux/rds.h | 4 +--
include/uapi/linux/rose.h | 8 ++---
include/uapi/linux/route.h | 2 +-
include/uapi/linux/rtc.h | 2 +-
include/uapi/linux/sctp.h | 18 +++++------
include/uapi/linux/sed-opal.h | 2 +-
include/uapi/linux/sem.h | 2 +-
include/uapi/linux/serial.h | 4 +--
include/uapi/linux/soundcard.h | 8 ++---
include/uapi/linux/taskstats.h | 2 +-
include/uapi/linux/virtio_net.h | 4 +--
include/uapi/linux/wireless.h | 4 +--
include/uapi/linux/xfrm.h | 26 ++++++++--------
include/uapi/mtd/mtd-abi.h | 2 +-
include/uapi/rdma/hfi/hfi1_ioctl.h | 2 +-
include/uapi/rdma/ib_user_ioctl_verbs.h | 2 +-
include/uapi/rdma/ib_user_mad.h | 2 +-
109 files changed, 205 insertions(+), 204 deletions(-)
On Tue, 16 Sep 2025, Arnd Bergmann wrote: > On Tue, Sep 16, 2025, at 02:16, Finn Thain wrote: > > > Packing uapi structures (and adopting -malign-int) sounds easier than > > the alternative, which might be to align certain internal kernel > > struct members, on a case-by-case basis, where doing so could be shown > > to improve performance on some architecture or other (while keeping > > -mno-align-int). > > > > Well, it's easy to find all the structs that belong to the uapi, but > > it's not easy to find all the internal kernel structs that describe > > MMIO registers. For -malign-int, both kinds of structs are a problem. > > Right, especially since those structure definitions are more likely to > be used on older drivers, there are probably some that do happen on > m68k. On the other hand, any driver that is portable to non-m68k targets > won't have this problem. > Yes, but only inasmuchas drivers are completely portable. Any data structure accessed or declared using #if conditional code would defeat that kind of reasoning. > I tried a trivial m68k defconfig build with "-Wpadded -malign-int" and > found 3021 instances of structure padding, compared to 2271 without > -malign-int. That is still something one could very reasonably go > through with 'vim -q output', as almost all of them are obviously > kernel-internal structures and not problematic. > So, 3021 is the number of structs potentially needing to be checked? This is not the kind of upper bound I consider feasible for careful manual editing (it's worse than I thought). We need the compiler to help. > A quick manual scan only shows a number of uapi structures but very few > drivers. There are a few hardware structures that are internally packed > but in a structure that has gets an extra two bytes of harmless padding > at the end (struct CUSTOM, struct amiga_hw_present, struct > atari_hw_present, struct TT_DMA, struct ppc_regs). I wouldn't assume that padding at the end is inconsequential. I think the analysis requires comparing object code. E.g. add the same padding explicitly to see whether it alters object code under -mno-align-int. > The only ones I could find that actually seemed suspicious are: > > arch/m68k/include/asm/atarihw.h:426:10: warning: padding struct to align > 'dst_address' [-Wpadded] arch/m68k/include/asm/openprom.h:76:13: > warning: padding struct to align 'boot_dev_ctrl' [-Wpadded] > drivers/net/ethernet/i825xx/82596.c:250:1: warning: padding struct size > to alignment boundary > > I'm sure there are a few more, but probably not a lot more. See > https://pastebin.com/Z2bjnD0G for the full list. I've also tried > annotating the ones that show up in defconfig, see diffstat below. > > Unfortunately I found no way of annotating a struct a whole to use the > traditional padding rules: "#pragma pack(push, 2)" lowers the alignment > of all struct members to two bytes, including those with an explicit > alignment like __aligned_u64. A struct-wide __attribute__((packed, > aligned(2))) seems even worse, as it makes all members inside of the > struck tightly packed and only aligns the struct itself. > > This means all misaligned members have to be individually marked as > __packed, unless someone comes up with another type of annotation. > Right. The trick will be to annotate the affected struct members by automatic program transformation. > > If better performance is to be had, my guess is that aligning atomic_t > > will get 80% of it (just an appeal to the Pareto principle, FWIW...) > > arch/m68k selects CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS for anything > other than Dragonball, so I would not expect much performance difference > at all, unless CASL on unaligned data ends up causing alignment traps as > it does on most architectures. > I think the answer to the performance question would depend on both choice of workload and choice of architecture. I haven't done any measurements on this patch series yet. > Arnd > > arch/m68k/atari/atakeyb.c | 2 +- > arch/m68k/include/asm/amigahw.h | 4 +-- > arch/m68k/include/asm/atarihw.h | 6 ++-- > arch/m68k/include/asm/mvme147hw.h | 2 +- > arch/m68k/include/asm/openprom.h | 2 +- > arch/m68k/include/uapi/asm/ptrace.h | 2 +- > arch/m68k/include/uapi/asm/sigcontext.h | 2 +- > arch/m68k/include/uapi/asm/stat.h | 4 +-- > drivers/net/ethernet/i825xx/82596.c | 4 +-- > include/uapi/asm-generic/termios.h | 2 +- > include/uapi/linux/acct.h | 2 +- > include/uapi/linux/ax25.h | 6 ++-- > include/uapi/linux/blktrace_api.h | 2 +- > include/uapi/linux/btrfs.h | 2 +- > include/uapi/linux/cdrom.h | 36 +++++++++++----------- > include/uapi/linux/dlm.h | 2 +- > include/uapi/linux/dqblk_xfs.h | 2 +- > include/uapi/linux/ethtool.h | 14 ++++----- > include/uapi/linux/fb.h | 6 ++-- > include/uapi/linux/fd.h | 6 ++-- > include/uapi/linux/filter.h | 2 +- > include/uapi/linux/hdlc/ioctl.h | 6 ++-- > include/uapi/linux/hiddev.h | 2 +- > include/uapi/linux/i2c-dev.h | 2 +- > include/uapi/linux/i2c.h | 2 +- > include/uapi/linux/if.h | 2 +- > include/uapi/linux/if_arcnet.h | 4 +-- > include/uapi/linux/if_bonding.h | 2 +- > include/uapi/linux/if_bridge.h | 14 ++++----- > include/uapi/linux/if_link.h | 2 +- > include/uapi/linux/if_plip.h | 2 +- > include/uapi/linux/if_pppox.h | 2 +- > include/uapi/linux/if_vlan.h | 2 +- > include/uapi/linux/inet_diag.h | 2 +- > include/uapi/linux/input.h | 4 +-- > include/uapi/linux/ip6_tunnel.h | 4 +-- > include/uapi/linux/kd.h | 2 +- > include/uapi/linux/llc.h | 2 +- > include/uapi/linux/loop.h | 2 +- > include/uapi/linux/mctp.h | 4 +-- > include/uapi/linux/mptcp.h | 2 +- > include/uapi/linux/mroute.h | 4 +-- > include/uapi/linux/mroute6.h | 6 ++-- > include/uapi/linux/msdos_fs.h | 2 +- > include/uapi/linux/msg.h | 6 ++-- > include/uapi/linux/mtio.h | 2 +- > include/uapi/linux/netfilter/ipset/ip_set.h | 4 +-- > include/uapi/linux/netfilter/nf_nat.h | 2 +- > include/uapi/linux/netfilter/x_tables.h | 8 ++--- > include/uapi/linux/netfilter/xt_HMARK.h | 2 +- > include/uapi/linux/netfilter/xt_TPROXY.h | 4 +-- > include/uapi/linux/netfilter/xt_connbytes.h | 2 +- > include/uapi/linux/netfilter/xt_connmark.h | 6 ++-- > include/uapi/linux/netfilter/xt_conntrack.h | 4 +-- > include/uapi/linux/netfilter/xt_esp.h | 2 +- > include/uapi/linux/netfilter/xt_hashlimit.h | 6 ++-- > include/uapi/linux/netfilter/xt_helper.h | 2 +- > include/uapi/linux/netfilter/xt_ipcomp.h | 2 +- > include/uapi/linux/netfilter/xt_iprange.h | 2 +- > include/uapi/linux/netfilter/xt_l2tp.h | 2 +- > include/uapi/linux/netfilter/xt_mac.h | 2 +- > include/uapi/linux/netfilter/xt_mark.h | 2 +- > include/uapi/linux/netfilter/xt_owner.h | 2 +- > include/uapi/linux/netfilter/xt_realm.h | 2 +- > include/uapi/linux/netfilter/xt_recent.h | 4 +-- > include/uapi/linux/netfilter/xt_set.h | 4 +-- > include/uapi/linux/netfilter/xt_statistic.h | 2 +- > include/uapi/linux/netfilter/xt_tcpmss.h | 2 +- > include/uapi/linux/netfilter/xt_tcpudp.h | 2 +- > include/uapi/linux/netfilter/xt_time.h | 2 +- > include/uapi/linux/netfilter/xt_u32.h | 6 ++-- > include/uapi/linux/netfilter_arp/arp_tables.h | 2 +- > include/uapi/linux/netfilter_arp/arpt_mangle.h | 2 +- > include/uapi/linux/netfilter_bridge/ebt_802_3.h | 2 +- > include/uapi/linux/netfilter_bridge/ebt_arp.h | 2 +- > include/uapi/linux/netfilter_bridge/ebt_arpreply.h | 2 +- > include/uapi/linux/netfilter_bridge/ebt_mark_m.h | 2 +- > include/uapi/linux/netfilter_bridge/ebt_nat.h | 2 +- > include/uapi/linux/netfilter_bridge/ebt_stp.h | 4 +-- > include/uapi/linux/netfilter_bridge/ebt_vlan.h | 2 +- > include/uapi/linux/netfilter_ipv4/ipt_ah.h | 2 +- > include/uapi/linux/netfilter_ipv6/ip6_tables.h | 2 +- > include/uapi/linux/netfilter_ipv6/ip6t_ah.h | 2 +- > include/uapi/linux/netfilter_ipv6/ip6t_frag.h | 2 +- > include/uapi/linux/netfilter_ipv6/ip6t_opts.h | 2 +- > include/uapi/linux/netfilter_ipv6/ip6t_rt.h | 2 +- > include/uapi/linux/netfilter_ipv6/ip6t_srh.h | 2 +- > include/uapi/linux/netrom.h | 2 +- > include/uapi/linux/nfs_idmap.h | 2 +- > include/uapi/linux/nfs_mount.h | 2 +- > include/uapi/linux/pkt_cls.h | 2 +- > include/uapi/linux/rds.h | 4 +-- > include/uapi/linux/rose.h | 8 ++--- > include/uapi/linux/route.h | 2 +- > include/uapi/linux/rtc.h | 2 +- > include/uapi/linux/sctp.h | 18 +++++------ > include/uapi/linux/sed-opal.h | 2 +- > include/uapi/linux/sem.h | 2 +- > include/uapi/linux/serial.h | 4 +-- > include/uapi/linux/soundcard.h | 8 ++--- > include/uapi/linux/taskstats.h | 2 +- > include/uapi/linux/virtio_net.h | 4 +-- > include/uapi/linux/wireless.h | 4 +-- > include/uapi/linux/xfrm.h | 26 ++++++++-------- > include/uapi/mtd/mtd-abi.h | 2 +- > include/uapi/rdma/hfi/hfi1_ioctl.h | 2 +- > include/uapi/rdma/ib_user_ioctl_verbs.h | 2 +- > include/uapi/rdma/ib_user_mad.h | 2 +- > 109 files changed, 205 insertions(+), 204 deletions(-) > Thanks for sending these results. I was looking into doing something similar using pahole but found that difficult. When I #include'd all the headers of interest, the thing doesn't build. I abandoned that approach and concluded that the way forward was to get the compiler to do the analysis (perhaps with a plug-in), during a normal kernel build, rather than during compilation of some contrived mess containing all the headers and all the structs. It would be sufficient to have a plug-in to list the sites of all members potentially needing annnotation. The list could be manually pared down and the actual annotating could be scripted. Note that this list would be shorter than a -Wpadded list, because there's a bunch of padding that's not relevant.
On Wed, Sep 17, 2025, at 04:14, Finn Thain wrote:
> On Tue, 16 Sep 2025, Arnd Bergmann wrote:
>> On Tue, Sep 16, 2025, at 02:16, Finn Thain wrote:
>>
>> Right, especially since those structure definitions are more likely to
>> be used on older drivers, there are probably some that do happen on
>> m68k. On the other hand, any driver that is portable to non-m68k targets
>> won't have this problem.
>>
>
> Yes, but only inasmuchas drivers are completely portable. Any data
> structure accessed or declared using #if conditional code would defeat
> that kind of reasoning.
Sure, but those are extremely rare. If there is a structure definition,
it's usually done in a way that describes the hardware and is already
densely packed. There are only a handful of compile-time checks for
CONFIG_M68K and __m68k__ in the kernel, and none of those are
around data structure definitions.
>> I tried a trivial m68k defconfig build with "-Wpadded -malign-int" and
>> found 3021 instances of structure padding, compared to 2271 without
>> -malign-int. That is still something one could very reasonably go
>> through with 'vim -q output', as almost all of them are obviously
>> kernel-internal structures and not problematic.
>>
>
> So, 3021 is the number of structs potentially needing to be checked? This
> is not the kind of upper bound I consider feasible for careful manual
> editing (it's worse than I thought). We need the compiler to help.
It's the number of add pads, so there are some structures that
show up many times in that list.
There would be significantly more than those 3021 if you include
drivers that are not in defconfig. An 'allmodconfig' build brings the
number up to 17000, but it's easy to quickly bring that number down
again by excluding stuff that is obviously specific to a random Arm
SoC. I would also exclude all of fs/* and most of net/* as being clearly
irrelevant here.
In a given file, there are likely many instances either in the
same struct or a set of related structs, where all can be discarded
after looking at the pattern in that file.
See the end of this email for a count by directory.
I think just doing the uapi headers is probably enough to avoid
accidentally introducing a new ABI when either the kernel or userspace
gets built with -malign-int, everything inside of the kernel is
at most a bug that can be fixed later.
>> A quick manual scan only shows a number of uapi structures but very few
>> drivers. There are a few hardware structures that are internally packed
>> but in a structure that has gets an extra two bytes of harmless padding
>> at the end (struct CUSTOM, struct amiga_hw_present, struct
>> atari_hw_present, struct TT_DMA, struct pcc_regs).
>
> I wouldn't assume that padding at the end is inconsequential. I think the
> analysis requires comparing object code. E.g. add the same padding
> explicitly to see whether it alters object code under -mno-align-int.
There is clearly a difference in uapi headers, especially when
it gets passed through an ioctl that encodes the size of the structure
in the command code.
For the structures I listed above, I can see no way it would actually
make a difference.
>> The only ones I could find that actually seemed suspicious are:
>>
>> arch/m68k/include/asm/atarihw.h:426:10: warning: padding struct to align
>> 'dst_address' [-Wpadded] arch/m68k/include/asm/openprom.h:76:13:
>> warning: padding struct to align 'boot_dev_ctrl' [-Wpadded]
>> drivers/net/ethernet/i825xx/82596.c:250:1: warning: padding struct size
>> to alignment boundary
>>
>> I'm sure there are a few more, but probably not a lot more. See
>> https://pastebin.com/Z2bjnD0G for the full list. I've also tried
>> annotating the ones that show up in defconfig, see diffstat below.
>>
>> Unfortunately I found no way of annotating a struct a whole to use the
>> traditional padding rules: "#pragma pack(push, 2)" lowers the alignment
>> of all struct members to two bytes, including those with an explicit
>> alignment like __aligned_u64. A struct-wide __attribute__((packed,
>> aligned(2))) seems even worse, as it makes all members inside of the
>> struck tightly packed and only aligns the struct itself.
>>
>> This means all misaligned members have to be individually marked as
>> __packed, unless someone comes up with another type of annotation.
>>
>
> Right. The trick will be to annotate the affected struct members by
> automatic program transformation.
I don't think automated transformation is going to work well here,
as you may not want the same approach in each case, depending
on what the code is:
- it may be enough to annotate a single member as packed in
order to make the entire structure compatible
- some structures may have lots of misaligned members, but
no holes, so a global __attribute__((packed, aligned(2))) on
that struct is cleaner
- if there are holes, some strategic additions of explicit
padding can be cleaner than annotating each misaligned
member.
- automation won't be able to tell whether a structure is
ABI relevant or not
- similarly, many files are not going to be interesting for
m68k. E.g. if drivers/infiniband has an ABI that is
different for -malign-int, that can likely be ignored
because nobody cares in practice.
> It would be sufficient to have a plug-in to list the sites of all members
> potentially needing annnotation. The list could be manually pared down and
> the actual annotating could be scripted. Note that this list would be
> shorter than a -Wpadded list, because there's a bunch of padding that's
> not relevant.
Yes, that should work to rule out many instances and to find out
exactly where packing attributes are required.
Arnd
2 arch/m68k/atari
5 arch/m68k/include/asm
39 block
1 block/fs
11 block/partitions
34 crypto
6 crypto/asymmetric_keys
10 drivers/accessibility/speakup
11 drivers/android
12 drivers/android/tests
15 drivers/ata
8 drivers/auxdisplay
11 drivers/base
5 drivers/base/firmware_loader
2 drivers/base/firmware_loader/builtin
10 drivers/base/regmap
2 drivers/base/test
2 drivers/bcma
18 drivers/block
2 drivers/block/aoe
7 drivers/block/drbd
4 drivers/block/null_blk
7 drivers/block/rnbd
2 drivers/block/zram
55 drivers/bluetooth
4 drivers/bus
2 drivers/bus/mhi/ep
4 drivers/bus/mhi/host
3 drivers/cdx
6 drivers/cdx/controller
3 drivers/char
8 drivers/char/hw_random
35 drivers/char/ipmi
11 drivers/char/tpm
1 drivers/char/tpm/eventlog
3 drivers/char/tpm/st33zp24
1 drivers/char/xillybus
48 drivers/clk
5 drivers/clk/actions
8 drivers/clk/bcm
8 drivers/clk/imx
6 drivers/clk/ingenic
11 drivers/clk/mediatek
24 drivers/clk/qcom
2 drivers/clk/ralink
21 drivers/clk/renesas
6 drivers/clk/samsung
1 drivers/clk/sifive
9 drivers/clk/socfpga
11 drivers/clk/sophgo
3 drivers/clk/sprd
2 drivers/clk/starfive
17 drivers/clk/sunxi-ng
1 drivers/clk/ti
1 drivers/clk/versatile
6 drivers/clk/visconti
2 drivers/clk/xilinx
43 drivers/clocksource
1 drivers/comedi
48 drivers/comedi/drivers
6 drivers/comedi/drivers/tests
9 drivers/counter
37 drivers/crypto
3 drivers/crypto/amlogic
4 drivers/crypto/aspeed
28 drivers/crypto/caam
17 drivers/crypto/ccree
3 drivers/crypto/hisilicon/sec
7 drivers/crypto/inside-secure
5 drivers/crypto/inside-secure/eip93
2 drivers/crypto/intel/ixp4xx
4 drivers/crypto/intel/keembay
3 drivers/crypto/marvell/cesa
7 drivers/crypto/qce
4 drivers/crypto/starfive
6 drivers/crypto/tegra
7 drivers/crypto/virtio
2 drivers/crypto/xilinx
4 drivers/dax
2 drivers/devfreq
1 drivers/devfreq/event
48 drivers/dma
3 drivers/dma-buf
1 drivers/dma-buf/heaps
7 drivers/dma/amd/qdma
4 drivers/dma/dw
3 drivers/dma/dw-axi-dmac
2 drivers/dma/lgm
3 drivers/dma/mediatek
5 drivers/dma/qcom
1 drivers/dma/sf-pdma
4 drivers/dma/sh
12 drivers/dma/stm32
14 drivers/dma/ti
27 drivers/dma/xilinx
5 drivers/dpll/zl3073x
14 drivers/extcon
11 drivers/firewire
8 drivers/firmware
35 drivers/firmware/arm_scmi
10 drivers/firmware/arm_scmi/transports
6 drivers/firmware/arm_scmi/vendors/imx
1 drivers/firmware/cirrus
2 drivers/firmware/google
9 drivers/firmware/imx
1 drivers/firmware/microchip
2 drivers/firmware/samsung
11 drivers/fpga
10 drivers/fsi
2 drivers/gnss
47 drivers/gpio
8 drivers/gpio/pinctrl
4 drivers/gpu/drm
12 drivers/gpu/drm/arm
15 drivers/gpu/drm/arm/display/komeda
1 drivers/gpu/drm/arm/display/komeda/d71
2 drivers/gpu/drm/atmel-hlcdc
40 drivers/gpu/drm/bridge
9 drivers/gpu/drm/bridge/adv7511
8 drivers/gpu/drm/bridge/analogix
9 drivers/gpu/drm/bridge/cadence
4 drivers/gpu/drm/bridge/imx
11 drivers/gpu/drm/bridge/synopsys
1 drivers/gpu/drm/clients
4 drivers/gpu/drm/display
5 drivers/gpu/drm/etnaviv
15 drivers/gpu/drm/exynos
3 drivers/gpu/drm/gud
3 drivers/gpu/drm/hisilicon/kirin
1 drivers/gpu/drm/imx/dc
11 drivers/gpu/drm/imx/dcss
2 drivers/gpu/drm/imx/ipuv3
6 drivers/gpu/drm/ingenic
9 drivers/gpu/drm/kmb
4 drivers/gpu/drm/lima
7 drivers/gpu/drm/logicvc
2 drivers/gpu/drm/mcde
39 drivers/gpu/drm/mediatek
17 drivers/gpu/drm/meson
2 drivers/gpu/drm/mxsfb
8 drivers/gpu/drm/omapdrm
41 drivers/gpu/drm/omapdrm/dss
41 drivers/gpu/drm/panel
2 drivers/gpu/drm/pl111
8 drivers/gpu/drm/renesas/rcar-du
2 drivers/gpu/drm/renesas/rz-du
22 drivers/gpu/drm/rockchip
1 drivers/gpu/drm/scheduler/tests
4 drivers/gpu/drm/sitronix
2 drivers/gpu/drm/solomon
5 drivers/gpu/drm/sprd
11 drivers/gpu/drm/sti
3 drivers/gpu/drm/stm
13 drivers/gpu/drm/sun4i
1 drivers/gpu/drm/sysfb
23 drivers/gpu/drm/tegra
13 drivers/gpu/drm/tests
1 drivers/gpu/drm/tests/sysfb
10 drivers/gpu/drm/tidss
3 drivers/gpu/drm/tiny
2 drivers/gpu/drm/ttm
4 drivers/gpu/drm/ttm/tests
3 drivers/gpu/drm/v3d
8 drivers/gpu/drm/virtio
3 drivers/gpu/drm/vkms
4 drivers/gpu/drm/vkms/tests
9 drivers/gpu/drm/xlnx
13 drivers/gpu/host1x
11 drivers/gpu/host1x/hw
13 drivers/gpu/ipu-v3
10 drivers/greybus
135 drivers/hid
4 drivers/hid/i2c-hid
5 drivers/hid/usbhid
1 drivers/hte
271 drivers/hwmon
5 drivers/hwmon/occ
4 drivers/hwmon/peci
19 drivers/hwmon/pmbus
8 drivers/hwtracing/intel_th
1 drivers/hwtracing/stm
9 drivers/i2c
110 drivers/i2c/busses
4 drivers/i2c/muxes
1 drivers/i2c/muxes/pinctrl
15 drivers/i3c/master
5 drivers/i3c/master/mipi-i3c-hci
2 drivers/iio
82 drivers/iio/accel
248 drivers/iio/adc
6 drivers/iio/addac
7 drivers/iio/amplifiers
2 drivers/iio/cdc
25 drivers/iio/chemical
1 drivers/iio/common/cros_ec_sensors
2 drivers/iio/common/ms_sensors
5 drivers/iio/common/ssp_sensors
80 drivers/iio/dac
2 drivers/iio/dummy
19 drivers/iio/frequency
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26 include/linux/spi
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17 include/linux/sunrpc
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24 include/media/i2c
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7 include/memory
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10 include/net/9p
90 include/net/bluetooth
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15 include/net/netns
20 include/net/nfc
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40 include/net/sctp
36 include/net/tc_act
29 include/pcmcia
101 include/rdma
89 include/scsi
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2 include/soc/microchip
17 include/soc/mscc
5 include/soc/tegra
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15 include/trace/events/../net/bridge
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2 include/uapi/drm
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3 include/uapi/linux/hdlc
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On Mon, 22 Sep 2025, Arnd Bergmann wrote: > > I don't think automated transformation is going to work well here, as > you may not want the same approach in each case, depending on what the > code is: > > - it may be enough to annotate a single member as packed in order to > make the entire structure compatible Right, and that's the only transformation I mentioned. > - some structures may have lots of misaligned members, but no holes, so > a global __attribute__((packed, aligned(2))) on that struct is cleaner That simplification should be amenable to further automation, if the first transformation is. > - if there are holes, some strategic additions of explicit padding can > be cleaner than annotating each misaligned member. It's a matter of taste. > - automation won't be able to tell whether a structure is ABI relevant > or not Right. That's why I said the list of struct members would need to be "manually pared down". But even that task may be too large to be feasible. > - similarly, many files are not going to be interesting for m68k. E.g. > if drivers/infiniband has an ABI that is different for -malign-int, > that can likely be ignored because nobody cares in practice. > Right. That's why I advocated running the plug-in on a "normal build" not a contrived mass of #includes and structs.
On Tue, Sep 16, 2025 at 02:37:21PM +0200, Arnd Bergmann wrote: > arch/m68k selects CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS for anything > other than Dragonball, so I would not expect much performance difference > at all, unless CASL on unaligned data ends up causing alignment traps > as it does on most architectures. I believe it depends on the exact CPU. The 68060 user manual has a section called "Emulating CAS2 and CAS Misaligned" discussing the handling of such instances through software emulation. The 68040 user manual doesn't have any similar section. I haven't looked at the exception handlers in the kernel to see if such cases are being handled. The documentation says it results in an unimplemented integer exception, and the handler has to manually lock the bus while performing normal memory operations. Brad Boyer flar@allandria.com
On Sep 16 2025, Brad Boyer wrote: > I believe it depends on the exact CPU. The 68060 user manual has a > section called "Emulating CAS2 and CAS Misaligned" discussing the > handling of such instances through software emulation. The 68040 > user manual doesn't have any similar section. The 68040 still handles them in hardware. -- Andreas Schwab, schwab@linux-m68k.org GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510 2552 DF73 E780 A9DA AEC1 "And now for something completely different."
Hi Finn,
CC Adrian,
On Tue, 16 Sept 2025 at 02:16, Finn Thain <fthain@linux-m68k.org> wrote:
> On Mon, 15 Sep 2025, Arnd Bergmann wrote:
> > On Mon, Sep 15, 2025, at 12:37, Finn Thain wrote:
> > > On Mon, 15 Sep 2025, Peter Zijlstra wrote:
> > >>
> > >> > When you do atomic operations on atomic_t or atomic64_t, (sizeof(long)
> > >> > - 1) probably doesn't make much sense. But atomic operations get used on
> > >> > scalar types (aside from atomic_t and atomic64_t) that don't have natural
> > >> > alignment. Please refer to the other thread about this:
> > >> > https://lore.kernel.org/all/ed1e0896-fd85-5101-e136-e4a5a37ca5ff@linux-m68k.org/
> > >>
> > >> Perhaps set ARCH_SLAB_MINALIGN ?
> > >
> > > That's not going to help much. The 850 byte offset of task_works into
> > > struct task_struct and the 418 byte offset of exit_state in struct
> > > task_struct are already misaligned.
> >
> > Has there been any progress on building m68k kernels with -mint-align?
>
> Not that I know of.
>
> > IIRC there are only a small number of uapi structures that need
> > __packed annotations to maintain the existing syscall ABI.
>
> Packing uapi structures (and adopting -malign-int) sounds easier than the
> alternative, which might be to align certain internal kernel struct
> members, on a case-by-case basis, where doing so could be shown to improve
> performance on some architecture or other (while keeping -mno-align-int).
indeed.
> Well, it's easy to find all the structs that belong to the uapi, but it's
> not easy to find all the internal kernel structs that describe MMIO
> registers. For -malign-int, both kinds of structs are a problem.
For structures under arch/m68k/include/asm/, just create a single
C file that calculates sizeof() of each structure, and compare the
generated code with and without -malign-int. Any differences should
be investigated, and attributed when needed.
For structures inside m68k-specific drivers, do something similar inside
those drivers ('git grep "struct\s*[a-zA-Z0-9_]*\s*{"' is your friend).
Most Amiga-specific drivers should be fine, as they were used on APUS
(PowerPC) before. I guess the same is true for some of the Mac-specific
drivers that are shared with PowerPC.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
On Tue, 16 Sep 2025, Geert Uytterhoeven wrote:
> On Tue, 16 Sept 2025 at 02:16, Finn Thain <fthain@linux-m68k.org> wrote:
> > On Mon, 15 Sep 2025, Arnd Bergmann wrote:
> > >
> > > Has there been any progress on building m68k kernels with
> > > -mint-align?
> >
> > Not that I know of.
> >
> > > IIRC there are only a small number of uapi structures that need
> > > __packed annotations to maintain the existing syscall ABI.
> >
> > Packing uapi structures (and adopting -malign-int) sounds easier than
> > the alternative, which might be to align certain internal kernel
> > struct members, on a case-by-case basis, where doing so could be shown
> > to improve performance on some architecture or other (while keeping
> > -mno-align-int).
>
> indeed.
>
Well, it "sounds easier". But I doubt that it really is easier.
> > Well, it's easy to find all the structs that belong to the uapi, but
> > it's not easy to find all the internal kernel structs that describe
> > MMIO registers. For -malign-int, both kinds of structs are a problem.
>
> For structures under arch/m68k/include/asm/, just create a single C file
> that calculates sizeof() of each structure, and compare the generated
> code with and without -malign-int. Any differences should be
> investigated, and attributed when needed.
>
> For structures inside m68k-specific drivers, do something similar inside
> those drivers ('git grep "struct\s*[a-zA-Z0-9_]*\s*{"' is your friend).
>
There's something to be said for adding static_assert() checks for the
structs that belong to all fixed interfaces. The patches to actually pack
of struct members could take place after that.
All of which is a lot of work, compared to specifying alignment for those
core kernel data structures where doing so improves performance. This
approach is platform neutral; it's not just m68k that benefits.
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