[PATCH v3]: perf/x86: store user space frame-pointer value on a sample

[Alexey Budankov]

Store user space frame-pointer value (BP register) into Perf trace 
on a sample for a process so the value becomes available when 
unwinding call stacks for functions gaining event samples.

Signed-off-by: Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx>
---
Changes in v3:
- adjusted comment regarding saved registers on system call after 
  recent KPTI improvements

Changes in v2:
- lifted restriction on frame pointer architecture so it's value is provided
  as for i386 as for x86_64 processes

MAINTAINERS file lacks references to appropriate folks for reviewing 
  changes at arch/x86/kernel/perf_regs.c so probably it makes sense to 
  update the file as well in this respect.
---
 arch/x86/kernel/perf_regs.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/perf_regs.c b/arch/x86/kernel/perf_regs.c
index e47b2dbbdef3..f54348a72add 100644
--- a/arch/x86/kernel/perf_regs.c
+++ b/arch/x86/kernel/perf_regs.c
@@ -151,17 +151,19 @@ void perf_get_regs_user(struct perf_regs *regs_user,
 	regs_user_copy->sp = user_regs->sp;
 	regs_user_copy->cs = user_regs->cs;
 	regs_user_copy->ss = user_regs->ss;
-
 	/*
-	 * Most system calls don't save these registers, don't report them.
+	 * Store user space frame-pointer value on sample
+	 * to facilitate stack unwinding for cases when
+	 * user space executable code has such support
+	 * enabled at compile time;
 	 */
+	regs_user_copy->bp = user_regs->bp;
+
 	regs_user_copy->bx = -1;
-	regs_user_copy->bp = -1;
 	regs_user_copy->r12 = -1;
 	regs_user_copy->r13 = -1;
 	regs_user_copy->r14 = -1;
 	regs_user_copy->r15 = -1;
-
 	/*
 	 * For this to be at all useful, we need a reasonable guess for
 	 * the ABI.  Be careful: we're in NMI context, and we're