[PATCH] powerpc/32: indirect function call use bctrl rather than blrl in ret_from_kernel_thread

Copied from commit 89bbe4c798bc ("powerpc/64: indirect function call
use bctrl rather than blrl in ret_from_kernel_thread")

blrl is not recommended to use as an indirect function call, as it may
corrupt the link stack predictor.

This is not a performance critical path but this should be fixed for
consistency.

Signed-off-by: Christophe Leroy <redacted>
---
 arch/powerpc/kernel/entry_32.S | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
index 0273a1349006..61fdd53cdd9a 100644
--- a/arch/powerpc/kernel/entry_32.S
+++ b/arch/powerpc/kernel/entry_32.S

@@ -161,10 +161,10 @@ ret_from_fork:
 ret_from_kernel_thread:
 	REST_NVGPRS(r1)
 	bl	schedule_tail
-	mtlr	r14
+	mtctr	r14
 	mr	r3,r15
 	PPC440EP_ERR42
-	blrl
+	bctrl
 	li	r3,0
 	b	ret_from_syscall
 

-- 
2.25.0

Christophe Leroy [off-list ref] writes:

Copied from commit 89bbe4c798bc ("powerpc/64: indirect function call
use bctrl rather than blrl in ret_from_kernel_thread")

blrl is not recommended to use as an indirect function call, as it may
corrupt the link stack predictor.

Do we know if any 32-bit CPUs have a link stack predictor or similar?

cheers

On Fri, Aug 20, 2021 at 10:15:11PM +1000, Michael Ellerman wrote:

Christophe Leroy [off-list ref] writes:

quoted

Copied from commit 89bbe4c798bc ("powerpc/64: indirect function call
use bctrl rather than blrl in ret_from_kernel_thread")

blrl is not recommended to use as an indirect function call, as it may
corrupt the link stack predictor.

Do we know if any 32-bit CPUs have a link stack predictor or similar?

74xx do.


Segher

On Fri, Aug 20, 2021 at 12:18:45PM -0500, Segher Boessenkool wrote:

On Fri, Aug 20, 2021 at 10:15:11PM +1000, Michael Ellerman wrote:

quoted

Christophe Leroy [off-list ref] writes:

quoted

Copied from commit 89bbe4c798bc ("powerpc/64: indirect function call
use bctrl rather than blrl in ret_from_kernel_thread")

blrl is not recommended to use as an indirect function call, as it may
corrupt the link stack predictor.

Do we know if any 32-bit CPUs have a link stack predictor or similar?

74xx do.

7450 and later, that is.  Will I ever get that right, sigh.


Segher

On Fri, 20 Aug 2021 05:16:05 +0000 (UTC), Christophe Leroy wrote:

Copied from commit 89bbe4c798bc ("powerpc/64: indirect function call
use bctrl rather than blrl in ret_from_kernel_thread")

blrl is not recommended to use as an indirect function call, as it may
corrupt the link stack predictor.

This is not a performance critical path but this should be fixed for
consistency.

[...]

Applied to powerpc/next.

[1/1] powerpc/32: indirect function call use bctrl rather than blrl in ret_from_kernel_thread
      https://git.kernel.org/powerpc/c/113ec9ccc8049c3772f0eab46b62c5d6654c09f7

cheers