Re: [PATCH v3 4/5] alpha: add HAVE_PERF_REGS and HAVE_PERF_USER_STACK_DUMP support

[sashiko-bot@kernel.org]

> Let perf sample registers and dump the user stack, which is what the
> perf tool needs for --call-graph dwarf.
> 
> The register indices follow the hardware numbering, so r0-r30 map
> directly to DWARF register numbers, with pc after them. r9-r15
> are callee-saved and live in switch_stack rather than pt_regs, so they
> cannot be sampled. Without r15 an unwinder cannot recover the CFA of a
> first frame that uses $15 as its frame pointer.
> 
> sp is not in pt_regs either: for user frames it comes from
> user_stack_pointer(), and for kernel frames (PERF_SAMPLE_REGS_INTR) it
> is the end of the trap frame, as in stacktrace.c.
> 
> Software events such as context switches and tracepoints hand perf a
> per-CPU pt_regs filled in by perf_fetch_caller_regs() instead of a
> trap frame. It is not on the stack and has no room for sp, so report
> sp as 0 there rather than the address of the per-CPU buffer.
> 
> Assisted-by: Claude:claude-opus-5-5
> Acked-by: Ian Rogers <irogers@google.com>
> Tested-by: Magnus Lindholm <linmag7@gmail.com>
> Reviewed-by: Magnus Lindholm <linmag7@gmail.com>
> Signed-off-by: Matt Turner <mattst88@gmail.com>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261018-alpha-perf-regs-v3-v3-0-52be85332c57@gmail.com?part=4