Re: [RFC PATCH v3 1/3] perf/core: Let an arch prepare and locate the user stack dump

[sashiko-bot@kernel.org]

> PERF_SAMPLE_STACK_USER copies the user stack as it is in memory,
> starting at the user stack pointer. Two things about sparc64 do not
> fit that.
> 
> The sampled window's %l and %i registers, including the frame pointer
> and return address an unwinder starts from, may still be in the
> register file: they only reach the stack when a window spills.
> perf_callchain_user() already handles this for the callchain by calling
> flushw_user() first; the user stack dump has no equivalent arch entry
> point.
> 
> And a 64-bit stack pointer is biased by 2047: the stack starts at
> %sp + 2047. Starting the dump at the register value spends a quarter of
> an 8K dump on memory below the frame, and gives no dump at all when that
> page has never been touched, as for a leaf frame at the deepest point a
> thread's stack has reached.
> 
> Add two hooks in the style of perf_arch_misc_flags():
> perf_arch_prepare_ustack(), a no-op by default, called from
> perf_prepare_sample() before the dump size is computed, and
> perf_arch_user_stack_pointer(), user_stack_pointer() by default, the
> address the dump starts at. Flushing in the arch PMU interrupt handler
> instead would not do: software events such as cpu-clock reach
> perf_event_overflow() without passing through it.
> 
> Signed-off-by: Stian Halseth <stian@itx.no>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20260923090429.2026529-1-stian@itx.no?part=1