Nohz full CPUs are not a desirable fallback target to run 32bits el0
applications. If present, prefer a set of housekeeping CPUs that can do
the job instead. Otherwise just don't support el0 32 bits. Should the
need arise, appropriate support can be introduced in the future.
Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
Documentation/arch/arm64/asymmetric-32bit.rst | 9 +++++++++
arch/arm64/kernel/cpufeature.c | 6 +++++-
2 files changed, 14 insertions(+), 1 deletion(-)
diff --git a/Documentation/arch/arm64/asymmetric-32bit.rst b/Documentation/arch/arm64/asymmetric-32bit.rst
index 64a0b505da7d..0c762cfc0f34 100644
--- a/Documentation/arch/arm64/asymmetric-32bit.rst
+++ b/Documentation/arch/arm64/asymmetric-32bit.rst
@@ -153,3 +153,12 @@ asymmetric system, a broken guest at EL1 could still attempt to execute
mode will return to host userspace with an ``exit_reason`` of
``KVM_EXIT_FAIL_ENTRY`` and will remain non-runnable until successfully
re-initialised by a subsequent ``KVM_ARM_VCPU_INIT`` operation.
+
+NOHZ FULL
+--------
+
+Nohz full CPUs are not a desirable fallback target to run 32bits el0
+applications. If present, a set of housekeeping CPUs that can do
+the job instead is preferred. Otherwise 32-bit EL0 is not supported.
+Should the need arise, appropriate support can be introduced in the
+future.
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 6ce71f444ed8..7ce1b8ab417f 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -75,6 +75,7 @@
#include <linux/cpu.h>
#include <linux/kasan.h>
#include <linux/percpu.h>
+#include <linux/sched/isolation.h>
#include <asm/cpu.h>
#include <asm/cpufeature.h>
@@ -3742,7 +3743,10 @@ static int enable_mismatched_32bit_el0(unsigned int cpu)
static int lucky_winner = -1;
struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
- bool cpu_32bit = id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0);
+ bool cpu_32bit = false;
+
+ if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0) && housekeeping_cpu(cpu, HK_TYPE_TICK))
+ cpu_32bit = true;
if (cpu_32bit) {
cpumask_set_cpu(cpu, cpu_32bit_el0_mask);
--
2.46.0On Wed, Dec 11, 2024 at 04:40:22PM +0100, Frederic Weisbecker wrote:
> Nohz full CPUs are not a desirable fallback target to run 32bits el0
> applications. If present, prefer a set of housekeeping CPUs that can do
> the job instead. Otherwise just don't support el0 32 bits. Should the
> need arise, appropriate support can be introduced in the future.
>
> Suggested-by: Will Deacon <will@kernel.org>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> ---
> Documentation/arch/arm64/asymmetric-32bit.rst | 9 +++++++++
> arch/arm64/kernel/cpufeature.c | 6 +++++-
Thanks, Frederic. A couple of comments below.
> 2 files changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/arch/arm64/asymmetric-32bit.rst b/Documentation/arch/arm64/asymmetric-32bit.rst
> index 64a0b505da7d..0c762cfc0f34 100644
> --- a/Documentation/arch/arm64/asymmetric-32bit.rst
> +++ b/Documentation/arch/arm64/asymmetric-32bit.rst
> @@ -153,3 +153,12 @@ asymmetric system, a broken guest at EL1 could still attempt to execute
> mode will return to host userspace with an ``exit_reason`` of
> ``KVM_EXIT_FAIL_ENTRY`` and will remain non-runnable until successfully
> re-initialised by a subsequent ``KVM_ARM_VCPU_INIT`` operation.
> +
> +NOHZ FULL
> +--------
> +
> +Nohz full CPUs are not a desirable fallback target to run 32bits el0
> +applications. If present, a set of housekeeping CPUs that can do
> +the job instead is preferred. Otherwise 32-bit EL0 is not supported.
> +Should the need arise, appropriate support can be introduced in the
> +future.
I think we can probably word this slightly better. How about something
more along these lines?
To avoid perturbing an adaptive-ticks CPU (specified using
``nohz_full=``) when a 32-bit task is forcefully migrated, these CPUs
are treated as 64-bit-only when support for asymmetric 32-bit systems
is enabled.
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 6ce71f444ed8..7ce1b8ab417f 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -75,6 +75,7 @@
> #include <linux/cpu.h>
> #include <linux/kasan.h>
> #include <linux/percpu.h>
> +#include <linux/sched/isolation.h>
>
> #include <asm/cpu.h>
> #include <asm/cpufeature.h>
> @@ -3742,7 +3743,10 @@ static int enable_mismatched_32bit_el0(unsigned int cpu)
> static int lucky_winner = -1;
>
> struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
> - bool cpu_32bit = id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0);
> + bool cpu_32bit = false;
> +
> + if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0) && housekeeping_cpu(cpu, HK_TYPE_TICK))
> + cpu_32bit = true;
I think it would be helpful to emit a diagnostic when a 32-bit CPU is
ignored because of the housekeeping check. e.g.
if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0)) {
if (!housekeeping_cpu(cpu, HK_TYPE_TICK))
pr_info("Treating adaptive-ticks CPU %u as 64-bit only\n", cpu);
else
cpu_32bit = true;
}
It's a bit of a bummer that this will fire on hardware that isn't
asymmetric, but I suppose that's easily resolved by not passing the
'allow_mismatched_32bit_el0' option in that case.
Will
Le Fri, Jan 03, 2025 at 03:20:44PM +0000, Will Deacon a écrit :
> On Wed, Dec 11, 2024 at 04:40:22PM +0100, Frederic Weisbecker wrote:
> > +NOHZ FULL
> > +--------
> > +
> > +Nohz full CPUs are not a desirable fallback target to run 32bits el0
> > +applications. If present, a set of housekeeping CPUs that can do
> > +the job instead is preferred. Otherwise 32-bit EL0 is not supported.
> > +Should the need arise, appropriate support can be introduced in the
> > +future.
>
> I think we can probably word this slightly better. How about something
> more along these lines?
>
> To avoid perturbing an adaptive-ticks CPU (specified using
> ``nohz_full=``) when a 32-bit task is forcefully migrated, these CPUs
> are treated as 64-bit-only when support for asymmetric 32-bit systems
> is enabled.
Indeed!
>
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index 6ce71f444ed8..7ce1b8ab417f 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -75,6 +75,7 @@
> > #include <linux/cpu.h>
> > #include <linux/kasan.h>
> > #include <linux/percpu.h>
> > +#include <linux/sched/isolation.h>
> >
> > #include <asm/cpu.h>
> > #include <asm/cpufeature.h>
> > @@ -3742,7 +3743,10 @@ static int enable_mismatched_32bit_el0(unsigned int cpu)
> > static int lucky_winner = -1;
> >
> > struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
> > - bool cpu_32bit = id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0);
> > + bool cpu_32bit = false;
> > +
> > + if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0) && housekeeping_cpu(cpu, HK_TYPE_TICK))
> > + cpu_32bit = true;
>
> I think it would be helpful to emit a diagnostic when a 32-bit CPU is
> ignored because of the housekeeping check. e.g.
>
>
> if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0)) {
> if (!housekeeping_cpu(cpu, HK_TYPE_TICK))
> pr_info("Treating adaptive-ticks CPU %u as 64-bit only\n", cpu);
> else
> cpu_32bit = true;
> }
>
>
> It's a bit of a bummer that this will fire on hardware that isn't
> asymmetric, but I suppose that's easily resolved by not passing the
> 'allow_mismatched_32bit_el0' option in that case.
Right! How is the following updated version?
---
From: Frederic Weisbecker <frederic@kernel.org>
Date: Fri, 25 Oct 2024 00:12:59 +0200
Subject: [PATCH 1/2] arm64: Exclude nohz_full CPUs from 32bits el0 support
Nohz full CPUs are not a desirable fallback target to run 32bits el0
applications. If present, prefer a set of housekeeping CPUs that can do
the job instead. Otherwise just don't support el0 32 bits. Should the
need arise, appropriate support can be introduced in the future.
Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
Documentation/arch/arm64/asymmetric-32bit.rst | 8 ++++++++
arch/arm64/kernel/cpufeature.c | 10 +++++++++-
2 files changed, 17 insertions(+), 1 deletion(-)
diff --git a/Documentation/arch/arm64/asymmetric-32bit.rst b/Documentation/arch/arm64/asymmetric-32bit.rst
index 64a0b505da7d..1ca2b359a907 100644
--- a/Documentation/arch/arm64/asymmetric-32bit.rst
+++ b/Documentation/arch/arm64/asymmetric-32bit.rst
@@ -153,3 +153,11 @@ asymmetric system, a broken guest at EL1 could still attempt to execute
mode will return to host userspace with an ``exit_reason`` of
``KVM_EXIT_FAIL_ENTRY`` and will remain non-runnable until successfully
re-initialised by a subsequent ``KVM_ARM_VCPU_INIT`` operation.
+
+NOHZ FULL
+---------
+
+To avoid perturbing an adaptive-ticks CPU (specified using
+``nohz_full=``) when a 32-bit task is forcefully migrated, these CPUs
+are treated as 64-bit-only when support for asymmetric 32-bit systems
+is enabled.
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 6ce71f444ed8..3c87659c14db 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -75,6 +75,7 @@
#include <linux/cpu.h>
#include <linux/kasan.h>
#include <linux/percpu.h>
+#include <linux/sched/isolation.h>
#include <asm/cpu.h>
#include <asm/cpufeature.h>
@@ -3742,7 +3743,14 @@ static int enable_mismatched_32bit_el0(unsigned int cpu)
static int lucky_winner = -1;
struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
- bool cpu_32bit = id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0);
+ bool cpu_32bit = false;
+
+ if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0)) {
+ if (!housekeeping_cpu(cpu, HK_TYPE_TICK))
+ pr_info("Treating adaptive-ticks CPU %u as 64-bit only\n", cpu);
+ else
+ cpu_32bit = true;
+ }
if (cpu_32bit) {
cpumask_set_cpu(cpu, cpu_32bit_el0_mask);
--
2.46.0
On Sun, Jan 05, 2025 at 12:18:40AM +0100, Frederic Weisbecker wrote:
> Le Fri, Jan 03, 2025 at 03:20:44PM +0000, Will Deacon a écrit :
> > I think it would be helpful to emit a diagnostic when a 32-bit CPU is
> > ignored because of the housekeeping check. e.g.
> >
> >
> > if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0)) {
> > if (!housekeeping_cpu(cpu, HK_TYPE_TICK))
> > pr_info("Treating adaptive-ticks CPU %u as 64-bit only\n", cpu);
> > else
> > cpu_32bit = true;
> > }
> >
> >
> > It's a bit of a bummer that this will fire on hardware that isn't
> > asymmetric, but I suppose that's easily resolved by not passing the
> > 'allow_mismatched_32bit_el0' option in that case.
>
> Right! How is the following updated version?
Thank you, that looks slightly better to me!
Acked-by: Will Deacon <will@kernel.org>
Will
> From: Frederic Weisbecker <frederic@kernel.org>
> Date: Fri, 25 Oct 2024 00:12:59 +0200
> Subject: [PATCH 1/2] arm64: Exclude nohz_full CPUs from 32bits el0 support
>
> Nohz full CPUs are not a desirable fallback target to run 32bits el0
> applications. If present, prefer a set of housekeeping CPUs that can do
> the job instead. Otherwise just don't support el0 32 bits. Should the
> need arise, appropriate support can be introduced in the future.
>
> Suggested-by: Will Deacon <will@kernel.org>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> ---
> Documentation/arch/arm64/asymmetric-32bit.rst | 8 ++++++++
> arch/arm64/kernel/cpufeature.c | 10 +++++++++-
> 2 files changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/arch/arm64/asymmetric-32bit.rst b/Documentation/arch/arm64/asymmetric-32bit.rst
> index 64a0b505da7d..1ca2b359a907 100644
> --- a/Documentation/arch/arm64/asymmetric-32bit.rst
> +++ b/Documentation/arch/arm64/asymmetric-32bit.rst
> @@ -153,3 +153,11 @@ asymmetric system, a broken guest at EL1 could still attempt to execute
> mode will return to host userspace with an ``exit_reason`` of
> ``KVM_EXIT_FAIL_ENTRY`` and will remain non-runnable until successfully
> re-initialised by a subsequent ``KVM_ARM_VCPU_INIT`` operation.
> +
> +NOHZ FULL
> +---------
> +
> +To avoid perturbing an adaptive-ticks CPU (specified using
> +``nohz_full=``) when a 32-bit task is forcefully migrated, these CPUs
> +are treated as 64-bit-only when support for asymmetric 32-bit systems
> +is enabled.
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 6ce71f444ed8..3c87659c14db 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -75,6 +75,7 @@
> #include <linux/cpu.h>
> #include <linux/kasan.h>
> #include <linux/percpu.h>
> +#include <linux/sched/isolation.h>
>
> #include <asm/cpu.h>
> #include <asm/cpufeature.h>
> @@ -3742,7 +3743,14 @@ static int enable_mismatched_32bit_el0(unsigned int cpu)
> static int lucky_winner = -1;
>
> struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
> - bool cpu_32bit = id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0);
> + bool cpu_32bit = false;
> +
> + if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0)) {
> + if (!housekeeping_cpu(cpu, HK_TYPE_TICK))
> + pr_info("Treating adaptive-ticks CPU %u as 64-bit only\n", cpu);
> + else
> + cpu_32bit = true;
> + }
>
> if (cpu_32bit) {
> cpumask_set_cpu(cpu, cpu_32bit_el0_mask);
> --
> 2.46.0
>
© 2016 - 2025 Red Hat, Inc.