[PATCH v3 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.

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@xxxxxxxxx>
Suggested-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
Reviewed-by: Magnus Lindholm <linmag7@xxxxxxxxx>
Acked-by: Ian Rogers <irogers@xxxxxxxxxx>
Signed-off-by: Matt Turner <mattst88@xxxxxxxxx>
---
 arch/alpha/include/asm/ftrace.h     | 5 ++---
 arch/alpha/include/asm/perf_event.h | 7 +++++++
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/alpha/include/asm/ftrace.h b/arch/alpha/include/asm/ftrace.h
index 7ec44134c804..5b75a294a7c9 100644
--- a/arch/alpha/include/asm/ftrace.h
+++ b/arch/alpha/include/asm/ftrace.h
@@ -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 */
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