Subject: Re: [PATCH] powerpc/64s: optimise syscall entry for virtual, relocatable case

From: Balbir Singh <[email-masked]>
Date: Tue, 20 Sep 2016 14:00:23 +1000

On 15/09/16 19:03, Nicholas Piggin wrote:
> The mflr r10 instruction was left over saving of lr when the code used
> lr to branch to system_call_entry from the exception handler. That was
> changed by 6a404806d to use the count register. The value is never used
> now, so mflr can be removed, and r10 can be used for storage rather than
> spilling to the SPR scratch register.
> 
> The scratch register spill causes a long pipeline stall due to the SPR
> read after write. This change brings getppid syscall cost from 406 to
> 376 cycles on POWER8. getppid for non-relocatable case is 371 cycles.
> 
> Signed-off-by: Nicholas Piggin <[email-masked]>
> ---
> 
>  arch/powerpc/kernel/exceptions-64s.S | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
> index df6d45e..2cdd64f 100644
> --- a/arch/powerpc/kernel/exceptions-64s.S
> +++ b/arch/powerpc/kernel/exceptions-64s.S
> @@ -63,15 +63,12 @@ END_FTR_SECTION_IFSET(CPU_FTR_REAL_LE)				\
>  	 * is volatile across system calls.
>  	 */
>  #define SYSCALL_PSERIES_2_DIRECT				\
> -	mflr	r10 ;						\
>  	ld	r12,PACAKBASE(r13) ; 				\
>  	LOAD_HANDLER(r12, system_call_entry) ;			\
>  	mtctr	r12 ;						\
>  	mfspr	r12,SPRN_SRR1 ;					\
> -	/* Re-use of r13... No spare regs to do this */	\
> -	li	r13,MSR_RI ;					\
> -	mtmsrd 	r13,1 ;						\
> -	GET_PACA(r13) ;	/* get r13 back */			\
> +	li	r10,MSR_RI ;					\
> +	mtmsrd 	r10,1 ;						\
>  	bctr ;
>  #else
>  	/* We can branch directly */
> 

The patch makes sense

Acked-by: Balbir Singh <[email-masked]>