Subject: Re: [PATCH 2/3] powerpc: Avoid load hit store in setup_sigcontext()

From: Michael Neuling <[email-masked]>
Date: Mon, 30 May 2016 09:14:23 +1000

On Sun, 2016-05-29 at 22:03 +1000, Anton Blanchard wrote:
> From: Anton Blanchard <[email-masked]>
> 
> In setup_sigcontext(), we set current->thread.vrsave then use it
> straight after. Since current is hidden from the compiler via inline
> assembly, it cannot optimise this and we end up with a load hit store.

Is setup_sigcontext() really a fast path we need to optimise?

Mikey

> Fix this by using a temporary.
> 
> Signed-off-by: Anton Blanchard <[email-masked]>
> ---
>  arch/powerpc/kernel/signal_64.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/signal_64.c
> b/arch/powerpc/kernel/signal_64.c
> index 2552079..7e49984 100644
> --- a/arch/powerpc/kernel/signal_64.c
> +++ b/arch/powerpc/kernel/signal_64.c
> @@ -104,6 +104,7 @@ static long setup_sigcontext(struct sigcontext __user
> *sc, struct pt_regs *regs,
>  	 */
>  #ifdef CONFIG_ALTIVEC
>  	elf_vrreg_t __user *v_regs = sigcontext_vmx_regs(sc);
> +	unsigned long vrsave;
>  #endif
>  	unsigned long msr = regs->msr;
>  	long err = 0;
> @@ -125,9 +126,13 @@ static long setup_sigcontext(struct sigcontext
> __user *sc, struct pt_regs *regs,
>  	/* We always copy to/from vrsave, it's 0 if we don't have or
> don't
>  	 * use altivec.
>  	 */
> -	if (cpu_has_feature(CPU_FTR_ALTIVEC))
> -		current->thread.vrsave = mfspr(SPRN_VRSAVE);
> -	err |= __put_user(current->thread.vrsave, (u32 __user
> *)&v_regs[33]);
> +	vrsave = 0;
> +	if (cpu_has_feature(CPU_FTR_ALTIVEC)) {
> +		vrsave = mfspr(SPRN_VRSAVE);
> +		current->thread.vrsave = vrsave;
> +	}
> +
> +	err |= __put_user(vrsave, (u32 __user *)&v_regs[33]);
>  #else /* CONFIG_ALTIVEC */
>  	err |= __put_user(0, &sc->v_regs);
>  #endif /* CONFIG_ALTIVEC */