Re: [PATCH 0/4] alpha: perf register sampling and user stack dumps

[Magnus Lindholm]

Hi Matt,

On Sun, Oct 11, 2026 at 10:16 PM Matt Turner <mattst88@xxxxxxxxx> wrote:
>
> This lets perf sample registers and dump the user stack on alpha, which
> perf record --call-graph dwarf needs to unwind user space.
>
> r9-r15 are not in pt_regs, so they cannot be sampled. An unwinder
> therefore cannot get past a first frame that uses $15 as its frame
> pointer (-O0 code or alloca).
>
> The perf tool side is a separate patch for perf-tools-next. libdw also
> needs alpha unwinding support, which I am sending to elfutils-devel.
>
> ---
> Matt Turner (4):
>       alpha: fix stale user_stack_pointer()
>       alpha: stop storing the user stack pointer in pt_regs
>       alpha: use user_stack_pointer() in KSTK_ESP() and dump_elf_thread()
>       alpha: add HAVE_PERF_REGS and HAVE_PERF_USER_STACK_DUMP support
>
>  .../features/perf/perf-regs/arch-support.txt       |  2 +-
>  .../features/perf/perf-stackdump/arch-support.txt  |  2 +-
>  arch/alpha/Kconfig                                 |  2 +
>  arch/alpha/include/asm/processor.h                 |  3 +-
>  arch/alpha/include/asm/ptrace.h                    |  7 +--
>  arch/alpha/include/uapi/asm/perf_regs.h            | 45 +++++++++++++++++
>  arch/alpha/include/uapi/asm/ptrace.h               |  2 +-
>  arch/alpha/kernel/Makefile                         |  1 +
>  arch/alpha/kernel/asm-offsets.c                    |  1 -
>  arch/alpha/kernel/entry.S                          |  5 --
>  arch/alpha/kernel/perf_regs.c                      | 56 ++++++++++++++++++++++
>  arch/alpha/kernel/process.c                        |  2 +-
>  arch/alpha/kernel/ptrace.c                         | 12 +++++
>  arch/alpha/kernel/traps.c                          |  8 ----
>  14 files changed, 122 insertions(+), 26 deletions(-)
> ---
> base-commit: 8f5dbfb93440d1aba216a1a34369a65654b94be9
> change-id: 20261001-alpha-perf-regs-v3-7e23508c1ca2
>
> Best regards,
> --
> Matt Turner <mattst88@xxxxxxxxx>
>

The four kernel patches apply cleanly on my for-next branch. I built and
booted the series on my ES40 (EV67/Tsunami), and tested software-clock,
page-fault and hardware-cycle sampling, register masks and user stack
dumps across all three online CPUs. The targeted tests passed.

I have a few review comments(I'll respond to the individual patches).
The separate perf patch records and decodes the samples, but
I cannot yet confirm DWARF unwinding: the installed elfutils 0.195 lacks
the required Alpha support. Could you point me to that patch as well?

Thanks,
Magnus