[PATCH v2 5/5] alpha: fill in pc for software event samples

[Matt Turner]

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.

Reported-by: Magnus Lindholm <linmag7@xxxxxxxxx>
Closes: https://lore.kernel.org/all/CA%2B%3DFv5S8-Zk8-LUpZnWiBfTGSQXthepmRc_-a9O%2BpEPmVW0mRA@xxxxxxxxxxxxxx/
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Matt Turner <mattst88@xxxxxxxxx>
---
 arch/alpha/include/asm/perf_event.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/alpha/include/asm/perf_event.h b/arch/alpha/include/asm/perf_event.h
index 5996e7a6757e..72536566014a 100644
--- a/arch/alpha/include/asm/perf_event.h
+++ b/arch/alpha/include/asm/perf_event.h
@@ -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 */

-- 
2.55.0