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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH] powerpc/32: indirect function call use bctrl rather
 than blrl in ret_from_kernel_thread
In-Reply-To: <91b1d242525307ceceec7ef6e832bfbacdd4501b.1629436472.git.christophe.leroy@csgroup.eu>
References: <91b1d242525307ceceec7ef6e832bfbacdd4501b.1629436472.git.christophe.leroy@csgroup.eu>
Date:   Fri, 20 Aug 2021 22:15:11 +1000
Message-ID: <871r6oe2i8.fsf@mpe.ellerman.id.au>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

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