I am adding HAVE_PERF_REGS and HAVE_PERF_USER_STACK_DUMP to sparc64, so
that perf record --call-graph dwarf and elfutils' eu-stackprof work
there. The sparc side (patch 2) is straightforward and follows parisc.
One thing does not fit in arch code, and I would like to get the shape
of that agreed before sending the rest.

The user stack dump copies the stack as it is in memory and assumes the
call chain is there. On sparc it may not be: the sampled register
window's %l/%i registers, which hold the frame pointer and return
address the unwinder starts from (the CFI after `save` defines the CFA
in terms of %i6), stay in the register file until a window spills. The
kernel already deals with this wherever it exposes user stack memory:
perf_callchain_user() on sparc calls flushw_user() before walking the
chain, and ptrace does the same. The stack dump has no arch entry
point where that could happen.

I looked for a sparc-only way and did not find a correct one:

- flushing in the sparc PMU interrupt handler misses software events
  (cpu-clock, tracepoints), which reach perf_event_overflow() without
  passing through it;
- perf_user_stack_pointer() is private to kernel/events/internal.h, so
  the arch cannot override it;
- perf_reg_abi() is called at the right time but is a query, and a
  flush as a side effect of it would be wrong.

So patch 1 adds a no-op hook in the style of perf_arch_misc_flags():
perf_arch_prepare_ustack(), called from perf_prepare_sample() when
PERF_SAMPLE_STACK_USER is requested and user regs exist. Patch 2 is the
sparc64 implementation and its user. Happy to take a different name or
placement.

Tested on an UltraSPARC T4-1 on 7.3-rc4 with a perf tool taught the
sparc registers (that patch, and the matching elfutils backend, follow
once the hook is settled): register values check out against known
contents, and --call-graph dwarf unwinds correctly for both cycles and
cpu-clock. perf stat/record/record -g are unchanged.

Link: https://github.com/sparclinux/issues/issues/99

Stian Halseth (2):
  perf/core: Let an arch prepare the user stack before it is dumped
  sparc64: Support PERF_SAMPLE_REGS_USER and PERF_SAMPLE_STACK_USER

 arch/sparc/Kconfig                      |  2 +
 arch/sparc/include/asm/perf_event.h     |  3 ++
 arch/sparc/include/uapi/asm/perf_regs.h | 33 +++++++++++++
 arch/sparc/kernel/Makefile              |  2 +-
 arch/sparc/kernel/perf_regs.c           | 65 +++++++++++++++++++++++++
 include/linux/perf_event.h              |  7 +++
 kernel/events/core.c                    |  2 +
 7 files changed, 113 insertions(+), 1 deletion(-)
 create mode 100644 arch/sparc/include/uapi/asm/perf_regs.h
 create mode 100644 arch/sparc/kernel/perf_regs.c

--
2.55.0

Hi Stian,

On Tue, Sep 22, 2026 at 4:38 PM Stian Halseth <stian@itx.no> wrote:
>
> I am adding HAVE_PERF_REGS and HAVE_PERF_USER_STACK_DUMP to sparc64, so
> that perf record --call-graph dwarf and elfutils' eu-stackprof work
> there. The sparc side (patch 2) is straightforward and follows parisc.
> One thing does not fit in arch code, and I would like to get the shape
> of that agreed before sending the rest.
>
> The user stack dump copies the stack as it is in memory and assumes the
> call chain is there. On sparc it may not be: the sampled register
> window's %l/%i registers, which hold the frame pointer and return
> address the unwinder starts from (the CFI after `save` defines the CFA
> in terms of %i6), stay in the register file until a window spills. The
> kernel already deals with this wherever it exposes user stack memory:
> perf_callchain_user() on sparc calls flushw_user() before walking the
> chain, and ptrace does the same. The stack dump has no arch entry
> point where that could happen.
>
> I looked for a sparc-only way and did not find a correct one:
>
> - flushing in the sparc PMU interrupt handler misses software events
>   (cpu-clock, tracepoints), which reach perf_event_overflow() without
>   passing through it;
> - perf_user_stack_pointer() is private to kernel/events/internal.h, so
>   the arch cannot override it;
> - perf_reg_abi() is called at the right time but is a query, and a
>   flush as a side effect of it would be wrong.
>
> So patch 1 adds a no-op hook in the style of perf_arch_misc_flags():
> perf_arch_prepare_ustack(), called from perf_prepare_sample() when
> PERF_SAMPLE_STACK_USER is requested and user regs exist. Patch 2 is the
> sparc64 implementation and its user. Happy to take a different name or
> placement.
>
> Tested on an UltraSPARC T4-1 on 7.3-rc4 with a perf tool taught the
> sparc registers (that patch, and the matching elfutils backend, follow
> once the hook is settled): register values check out against known
> contents, and --call-graph dwarf unwinds correctly for both cycles and
> cpu-clock. perf stat/record/record -g are unchanged.
>
> Link: https://github.com/sparclinux/issues/issues/99
>
> Stian Halseth (2):
>   perf/core: Let an arch prepare the user stack before it is dumped
>   sparc64: Support PERF_SAMPLE_REGS_USER and PERF_SAMPLE_STACK_USER
>
>  arch/sparc/Kconfig                      |  2 +
>  arch/sparc/include/asm/perf_event.h     |  3 ++
>  arch/sparc/include/uapi/asm/perf_regs.h | 33 +++++++++++++
>  arch/sparc/kernel/Makefile              |  2 +-
>  arch/sparc/kernel/perf_regs.c           | 65 +++++++++++++++++++++++++
>  include/linux/perf_event.h              |  7 +++
>  kernel/events/core.c                    |  2 +
>  7 files changed, 113 insertions(+), 1 deletion(-)
>  create mode 100644 arch/sparc/include/uapi/asm/perf_regs.h
>  create mode 100644 arch/sparc/kernel/perf_regs.c
>
> --
> 2.55.0
>


Very nice series, thanks for working on this!

I built and booted the series on a sparc64 box and haven't
noticed any regressions so far.

I also smoke-tested the new interfaces from 64-bit userspace with a
small standalone C program using perf_event_open() and software
cpu-clock events. The register-only, stack-only, and combined
register/stack tests all passed. The test does not use tools/perf or
request PERF_SAMPLE_CALLCHAIN. I haven't tested end-to-end DWARF
unwinding yet.

I see that you plan to post the userspace changes once the hook is
settled. Could you include the tools/perf support in the next revision,
or post it as a follow-up or linked companion series? Having that
available, along with a link to the matching elfutils changes, would
make testing easier and let us exercise the intended DWARF unwinding
use case directly.

Also, could you update the sparc entries from TODO to ok in:

  Documentation/features/perf/perf-regs/arch-support.txt
  Documentation/features/perf/perf-stackdump/arch-support.txt

Those updates would fit naturally in patch 2 alongside the
HAVE_PERF_REGS and HAVE_PERF_USER_STACK_DUMP selections.

Thanks,
Magnus

Hi Magnus, thanks for the testing and feedback.

Almost done with a v2.

It has some minor changes based on the AI bot feedback, and my comments
to you below.


On Tue, 2026-09-22 at 19:04 +0200, Magnus Lindholm wrote:
> 
> Very nice series, thanks for working on this!
> 
> I built and booted the series on a sparc64 box and haven't
> noticed any regressions so far.
> 
> I also smoke-tested the new interfaces from 64-bit userspace with a
> small standalone C program using perf_event_open() and software
> cpu-clock events. The register-only, stack-only, and combined
> register/stack tests all passed. The test does not use tools/perf or
> request PERF_SAMPLE_CALLCHAIN. I haven't tested end-to-end DWARF
> unwinding yet.
> 
> I see that you plan to post the userspace changes once the hook is
> settled. Could you include the tools/perf support in the next
> revision,
> or post it as a follow-up or linked companion series? Having that
> available, along with a link to the matching elfutils changes, would
> make testing easier and let us exercise the intended DWARF unwinding
> use case directly.
Yes, I originally planned to post tools/perf after I got some feedback
on the first patches. Especially since it touches some generic code
that I'm not too familiar with, and I half expect that I have to
rewrite some of it based on maintainer feedback :-)

But since its "just" a RFC patch at this point, I guess I can post the
full series to allow end-to-end testing. 

I have mainly focused on the first patches, and the rest was done a bit
faster as a means to validate the "core" changes. It works (TM). I'm
looking through it now, but I probably make a second thorough review of
the entire final series, once the I know what changes I need to make.

I will publish and link to the elfutils changes as well.
> 
> Also, could you update the sparc entries from TODO to ok in:
> 
>   Documentation/features/perf/perf-regs/arch-support.txt
>   Documentation/features/perf/perf-stackdump/arch-support.txt
> 
> Those updates would fit naturally in patch 2 alongside the
> HAVE_PERF_REGS and HAVE_PERF_USER_STACK_DUMP selections.
Yes, agree.



-- 
Best regards
Stian Halseth
​