[PATCH 1/2] powerpc/pseries: Fix hcall tracepoints with JUMP_LABEL=n

[PATCH 1/2] powerpc/pseries: Fix hcall tracepoints with JUMP_LABEL=n

[Nicholas Piggin]

With JUMP_LABEL=n, hcall_tracepoint_refcount's address is being tested
instead of its value. This results in the tracing slowpath always being
taken unnecessarily.

Fixes: 9a10ccb29c0a2 ("powerpc/pseries: move hcall_tracepoint_refcount out of .toc")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/platforms/pseries/hvCall.S | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/powerpc/platforms/pseries/hvCall.S b/arch/powerpc/platforms/pseries/hvCall.S
index 35254ac7af5e..ca0674b0b683 100644
--- a/arch/powerpc/platforms/pseries/hvCall.S
+++ b/arch/powerpc/platforms/pseries/hvCall.S
@@ -91,6 +91,7 @@ BEGIN_FTR_SECTION;						\
 	b	1f;						\
 END_FTR_SECTION(0, 1);						\
 	LOAD_REG_ADDR(r12, hcall_tracepoint_refcount) ;		\
+	ld	r12,0(r12);					\
 	std	r12,32(r1);					\
 	cmpdi	r12,0;						\
 	bne-	LABEL;						\
-- 
2.40.1

[PATCH 2/2] powerpc/pseries: Remove unused hcall tracing instruction

[Nicholas Piggin]

When JUMP_LABEL=n, the tracepoint refcount test in the pre-call stores
the refcount value to the stack, so the same value can be used for the
post-call (presumably to avoid racing with the value concurrently
changing).

On little-endian (ELFv2) that might have just worked by luck, because
32(r1) is STK_PARAM(R3) there and so the value save gets clobbered by
the tracing code when it's non-zero, but fortunately r3 is the hcall
number and 0 is an invalid hcall number so it should get clobbered by
another non-zero value. In any case, commit cc1adb5f32557
("powerpc/pseries: Use jump labels for hcall tracepoints") removed the
code that actually used the value stored, so now it's just dead code.

It's fragile to be storing to the stack like this, and confusing. Better
remove it.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/platforms/pseries/hvCall.S | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/powerpc/platforms/pseries/hvCall.S b/arch/powerpc/platforms/pseries/hvCall.S
index ca0674b0b683..bae45b358a09 100644
--- a/arch/powerpc/platforms/pseries/hvCall.S
+++ b/arch/powerpc/platforms/pseries/hvCall.S
@@ -92,7 +92,6 @@ BEGIN_FTR_SECTION;						\
 END_FTR_SECTION(0, 1);						\
 	LOAD_REG_ADDR(r12, hcall_tracepoint_refcount) ;		\
 	ld	r12,0(r12);					\
-	std	r12,32(r1);					\
 	cmpdi	r12,0;						\
 	bne-	LABEL;						\
 1:
-- 
2.40.1

Re: [PATCH 1/2] powerpc/pseries: Fix hcall tracepoints with JUMP_LABEL=n

[Michael Ellerman]

On Tue, 09 May 2023 19:15:59 +1000, Nicholas Piggin wrote:
> With JUMP_LABEL=n, hcall_tracepoint_refcount's address is being tested
> instead of its value. This results in the tracing slowpath always being
> taken unnecessarily.
> 
> 

Applied to powerpc/next.

[1/2] powerpc/pseries: Fix hcall tracepoints with JUMP_LABEL=n
      https://git.kernel.org/powerpc/c/750bd41aeaeb1f0e0128aa4f8fcd6dd759713641
[2/2] powerpc/pseries: Remove unused hcall tracing instruction
      https://git.kernel.org/powerpc/c/61d7ebe0376e2640ba77be16e186b1a6c77eb3f7

cheers