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

[Stian Halseth <stian@itx.no>]

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>
---
 include/linux/perf_event.h | 11 +++++++++++
 kernel/events/core.c       |  2 ++
 kernel/events/internal.h   |  2 +-
 3 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 5842552294c1..49aa22db43c0 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1815,6 +1815,17 @@ extern unsigned long perf_instruction_pointer(struct perf_event *event,
 #ifndef perf_arch_bpf_user_pt_regs
 # define perf_arch_bpf_user_pt_regs(regs) regs
 #endif
+/*
+ * Called before the user stack of the current task is dumped, for an arch
+ * that still holds part of the user's stack state in registers.
+ */
+#ifndef perf_arch_prepare_ustack
+static inline void perf_arch_prepare_ustack(void) { }
+#endif
+/* Where the user stack dump starts, for an arch with a biased stack pointer. */
+#ifndef perf_arch_user_stack_pointer
+# define perf_arch_user_stack_pointer(regs)	user_stack_pointer(regs)
+#endif
 
 #ifndef perf_arch_guest_misc_flags
 static inline unsigned long perf_arch_guest_misc_flags(struct pt_regs *regs)
diff --git a/kernel/events/core.c b/kernel/events/core.c
index db7b76d6b68a..90fb35c7d279 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8718,6 +8718,8 @@ void perf_prepare_sample(struct perf_sample_data *data,
 		u16 header_size = perf_sample_data_size(data, event);
 		u16 size = sizeof(u64);
 
+		if (data->regs_user.regs)
+			perf_arch_prepare_ustack();
 		stack_size = perf_sample_ustack_size(stack_size, header_size,
 						     data->regs_user.regs);
 
diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index c03c4f2eea57..01b5f53d01d7 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -236,7 +236,7 @@ static inline bool arch_perf_have_user_stack_dump(void)
 	return true;
 }
 
-#define perf_user_stack_pointer(regs) user_stack_pointer(regs)
+#define perf_user_stack_pointer(regs) perf_arch_user_stack_pointer(regs)
 #else
 static inline bool arch_perf_have_user_stack_dump(void)
 {
-- 
2.55.0