Re: [PATCH] powerpc: Clear RI and EE at the same time in system call exit

[Scott Wood <scottwood@freescale.com>]

On 05/29/2012 06:20 AM, Anton Blanchard wrote:
> 
> In system call exit we currently clear RI and EE separately. An
> mtmsrd is a slow operation and we can save cycles by doing it all
> in one go.
> 
> This does complicate things a bit - we have to be careful to restore
> RI if we branch out before returning to userspace.
> 
> On a POWER7 with virtual cputime disabled this patch improves the
> null system call by 7%.
> 
> Signed-off-by: Anton Blanchard <anton@samba.org>
> ---
> 
> Index: linux-build/arch/powerpc/kernel/entry_64.S
> ===================================================================
> --- linux-build.orig/arch/powerpc/kernel/entry_64.S	2012-05-28 18:23:33.374451416 +1000
> +++ linux-build/arch/powerpc/kernel/entry_64.S	2012-05-29 21:18:22.280934940 +1000
> @@ -197,7 +197,16 @@ syscall_exit:
>  	wrteei	0
>  #else
>  	ld	r10,PACAKMSR(r13)
> -	mtmsrd	r10,1
> +	/*
> +	 * For performance reasons we clear RI the same time that we
> +	 * clear EE. We only need to clear RI just before we restore r13
> +	 * below, but batching it with EE saves us one expensive mtmsrd call.
> +	 * We have to be careful to restore RI if we branch anywhere from
> +	 * here (eg syscall_exit_work).
> +	 */
> +	li	r9,MSR_RI
> +	andc	r11,r10,r9
> +	mtmsrd	r11,1
>  #endif /* CONFIG_PPC_BOOK3E */
>  
>  	ld	r9,TI_FLAGS(r12)
> @@ -214,17 +223,6 @@ BEGIN_FTR_SECTION
>  END_FTR_SECTION_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS)
>  	andi.	r6,r8,MSR_PR
>  	ld	r4,_LINK(r1)
> -	/*
> -	 * Clear RI before restoring r13.  If we are returning to
> -	 * userspace and we take an exception after restoring r13,
> -	 * we end up corrupting the userspace r13 value.
> -	 */
> -#ifdef CONFIG_PPC_BOOK3S
> -	/* No MSR:RI on BookE */
> -	li	r12,MSR_RI
> -	andc	r11,r10,r12
> -	mtmsrd	r11,1			/* clear MSR.RI */
> -#endif /* CONFIG_PPC_BOOK3S */
>  
>  	beq-	1f
>  	ACCOUNT_CPU_USER_EXIT(r11, r12)
> @@ -271,6 +269,7 @@ syscall_enosys:
>  	b	syscall_exit
>  	
>  syscall_exit_work:
> +	mtmsrd	r10,1		/* Restore RI */

That mtmsrd needs an #ifdef CONFIG_PPC_BOOK3S

-Scott