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* [PATCH] powerpc/32: indirect function call use bctrl rather than blrl in ret_from_kernel_thread
@ 2021-08-20  5:16 Christophe Leroy
  2021-08-20 12:15 ` Michael Ellerman
  2021-08-27 13:15 ` Michael Ellerman
  0 siblings, 2 replies; 5+ messages in thread
From: Christophe Leroy @ 2021-08-20  5:16 UTC (permalink / raw)
  To: Benjamin Herrenschmidt, Paul Mackerras, Michael Ellerman
  Cc: linux-kernel, linuxppc-dev

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 <christophe.leroy@csgroup.eu>
---
 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


^ permalink raw reply	[flat|nested] 5+ messages in thread

* Re: [PATCH] powerpc/32: indirect function call use bctrl rather than blrl in ret_from_kernel_thread
  2021-08-20  5:16 [PATCH] powerpc/32: indirect function call use bctrl rather than blrl in ret_from_kernel_thread Christophe Leroy
@ 2021-08-20 12:15 ` Michael Ellerman
  2021-08-20 17:18   ` Segher Boessenkool
  2021-08-27 13:15 ` Michael Ellerman
  1 sibling, 1 reply; 5+ messages in thread
From: Michael Ellerman @ 2021-08-20 12:15 UTC (permalink / raw)
  To: Christophe Leroy, Benjamin Herrenschmidt, Paul Mackerras
  Cc: linux-kernel, linuxppc-dev

Christophe Leroy <christophe.leroy@csgroup.eu> 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

> This is not a performance critical path but this should be fixed for
> consistency.
>
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
>  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

^ permalink raw reply	[flat|nested] 5+ messages in thread

* Re: [PATCH] powerpc/32: indirect function call use bctrl rather than blrl in ret_from_kernel_thread
  2021-08-20 12:15 ` Michael Ellerman
@ 2021-08-20 17:18   ` Segher Boessenkool
  2021-08-20 18:12     ` Segher Boessenkool
  0 siblings, 1 reply; 5+ messages in thread
From: Segher Boessenkool @ 2021-08-20 17:18 UTC (permalink / raw)
  To: Michael Ellerman
  Cc: Christophe Leroy, Benjamin Herrenschmidt, Paul Mackerras,
	linuxppc-dev, linux-kernel

On Fri, Aug 20, 2021 at 10:15:11PM +1000, Michael Ellerman wrote:
> Christophe Leroy <christophe.leroy@csgroup.eu> 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?

74xx do.


Segher

^ permalink raw reply	[flat|nested] 5+ messages in thread

* Re: [PATCH] powerpc/32: indirect function call use bctrl rather than blrl in ret_from_kernel_thread
  2021-08-20 17:18   ` Segher Boessenkool
@ 2021-08-20 18:12     ` Segher Boessenkool
  0 siblings, 0 replies; 5+ messages in thread
From: Segher Boessenkool @ 2021-08-20 18:12 UTC (permalink / raw)
  To: Michael Ellerman; +Cc: Paul Mackerras, linuxppc-dev, linux-kernel

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:
> > Christophe Leroy <christophe.leroy@csgroup.eu> 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?
> 
> 74xx do.

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


Segher

^ permalink raw reply	[flat|nested] 5+ messages in thread

* Re: [PATCH] powerpc/32: indirect function call use bctrl rather than blrl in ret_from_kernel_thread
  2021-08-20  5:16 [PATCH] powerpc/32: indirect function call use bctrl rather than blrl in ret_from_kernel_thread Christophe Leroy
  2021-08-20 12:15 ` Michael Ellerman
@ 2021-08-27 13:15 ` Michael Ellerman
  1 sibling, 0 replies; 5+ messages in thread
From: Michael Ellerman @ 2021-08-27 13:15 UTC (permalink / raw)
  To: Michael Ellerman, Christophe Leroy, Paul Mackerras,
	Benjamin Herrenschmidt
  Cc: linuxppc-dev, linux-kernel

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

^ permalink raw reply	[flat|nested] 5+ messages in thread

end of thread, other threads:[~2021-08-27 13:22 UTC | newest]

Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-08-20  5:16 [PATCH] powerpc/32: indirect function call use bctrl rather than blrl in ret_from_kernel_thread Christophe Leroy
2021-08-20 12:15 ` Michael Ellerman
2021-08-20 17:18   ` Segher Boessenkool
2021-08-20 18:12     ` Segher Boessenkool
2021-08-27 13:15 ` Michael Ellerman

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