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alpha: fill in pc for software event samples

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Context switch and tracepoint events have no trap frame, so perf builds
one with perf_fetch_caller_regs(). Alpha does not define
perf_arch_fetch_caller_regs(), which leaves that pt_regs zeroed, and
these samples report pc as 0.

Record the caller's address as pc. The zero ps already marks the frame
as kernel mode, and pt_regs has nowhere to keep sp, so pc is all that
can be filled in.

perf_fetch_caller_regs() gets that address from CALLER_ADDR0, which
alpha defines as 0 when CONFIG_FRAME_POINTER is disabled. Finding the
level zero return address does not need a frame pointer, so make
alpha_ftrace_return_address0() available in both configurations. Only
ftrace_return_address(n) still depends on frame pointers.

Reported-by: Magnus Lindholm <linmag7@gmail.com>
Closes: https://lore.kernel.org/all/CA%2B%3DFv5S8-Zk8-LUpZnWiBfTGSQXthepmRc_-a9O%2BpEPmVW0mRA@mail.gmail.com/
Suggested-by: Magnus Lindholm <linmag7@gmail.com>
Assisted-by: Claude:claude-opus-5-5
Reviewed-by: Magnus Lindholm <linmag7@gmail.com>
Acked-by: Ian Rogers <irogers@google.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>
Tested-by: Magnus Lindholm <linmag7@gmail.com>
Link: https://lore.kernel.org/r/20261018-alpha-perf-regs-v3-v3-5-52be85332c57@gmail.com
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
This commit is contained in:
Matt Turner 2026-10-18 10:22:05 -04:00 committed by Magnus Lindholm
parent 9280e99d87
commit 1a85a03fc2
2 changed files with 9 additions and 3 deletions
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5
arch/alpha/include/asm/ftrace.h
View File

@ -2,8 +2,6 @@
#ifndef _ASM_ALPHA_FTRACE_H
#define _ASM_ALPHA_FTRACE_H
#ifdef CONFIG_FRAME_POINTER
static void *alpha_ftrace_return_address0(void)
noinline notrace;
static void *alpha_ftrace_return_address0(void)
@ -13,6 +11,8 @@ static void *alpha_ftrace_return_address0(void)
#define ftrace_return_address0 alpha_ftrace_return_address0()
#ifdef CONFIG_FRAME_POINTER
/*
* __builtin_return_address() requires a constant integer argument.
* Keep this as a macro so the value is seen at the callsite.
@ -21,7 +21,6 @@ static void *alpha_ftrace_return_address0(void)
#else /* !CONFIG_FRAME_POINTER */
#define ftrace_return_address0 0UL
#define ftrace_return_address(n) ((void)(n), 0UL)
#endif /* CONFIG_FRAME_POINTER */

7
arch/alpha/include/asm/perf_event.h
View File

@ -1,4 +1,11 @@
#ifndef __ASM_ALPHA_PERF_EVENT_H
#define __ASM_ALPHA_PERF_EVENT_H
/*
* Software events have no trap frame, so perf_fetch_caller_regs() fills
* in a zeroed pt_regs. A zero ps already means kernel mode; only pc is
* missing.
*/
#define perf_arch_fetch_caller_regs(regs, __ip) ((regs)->pc = (__ip))
#endif /* __ASM_ALPHA_PERF_EVENT_H */