2021-08-20 05:17:02

by Christophe Leroy

Subject: [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 <[email protected]>
---
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

2021-08-20 12:19:08

by Michael Ellerman

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

Christophe Leroy <[email protected]> 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 <[email protected]>
> ---
> 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

2021-08-20 17:26:59

by Segher Boessenkool

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

On Fri, Aug 20, 2021 at 10:15:11PM +1000, Michael Ellerman wrote:
> Christophe Leroy <[email protected]> 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

2021-08-20 18:18:17

by Segher Boessenkool

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

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 <[email protected]> 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

2021-08-27 13:25:42

by Michael Ellerman

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

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