[PATCH] powerpc/64s: exception optimise MSR handling

[Michael Ellerman]

Nicholas Piggin <npiggin at gmail.com> writes:

> mtmsrd with L=1 only affects MSR_EE and MSR_RI bits, and we always
> know what state those bits are, so the kernel MSR does not need to be
> loaded when modifying them.
>
> mtmsrd is often in the critical execution path, so avoiding dependency
> on even L1 load is noticable. On a POWER8 this saves about 3 cycles
> from the syscall path, and possibly a few from other exception returns
> (not measured).

This looks good in principle.

My worry is that these code paths have lots of assumptions about what's
left in registers, so we may have a path somewhere which expects rX to
contain PACAKMSR. Hence the review below ...

> diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
> index 6b8bc0d..585b9ca 100644
> --- a/arch/powerpc/kernel/entry_64.S
> +++ b/arch/powerpc/kernel/entry_64.S
> @@ -139,7 +139,7 @@ END_FW_FTR_SECTION_IFSET(FW_FEATURE_SPLPAR)
>  #ifdef CONFIG_PPC_BOOK3E
>  	wrteei	1
>  #else
> -	ld	r11,PACAKMSR(r13)
> +	li	r11,MSR_RI
>  	ori	r11,r11,MSR_EE
>  	mtmsrd	r11,1
>  #endif /* CONFIG_PPC_BOOK3E */

	/* We do need to set SOFTE in the stack frame or the return
	 * from interrupt will be painful
	 */
	li	r10,1
	std	r10,SOFTE(r1)

	CURRENT_THREAD_INFO(r11, r1)

So that's one OK. r11 isn't used again until its clobbered here.


> @@ -195,7 +195,6 @@ system_call:			/* label this so stack traces look sane */

#ifdef CONFIG_PPC_BOOK3S
	/* No MSR:RI on BookE */
	andi.	r10,r8,MSR_RI
	beq-	unrecov_restore
#endif

So at this point r10 == MSR_RI, otherwise we would have taken the branch.

	/*
	 * Disable interrupts so current_thread_info()->flags can't change,
	 * and so that we don't get interrupted after loading SRR0/1.
	 */
>  #ifdef CONFIG_PPC_BOOK3E
>  	wrteei	0
>  #else
> -	ld	r10,PACAKMSR(r13)
>  	/*
>  	 * 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
> +	li	r11,0
>  	mtmsrd	r11,1
>  #endif /* CONFIG_PPC_BOOK3E */

	ld	r9,TI_FLAGS(r12)
	li	r11,-MAX_ERRNO
	andi.	r0,r9,(_TIF_SYSCALL_DOTRACE|_TIF_SINGLESTEP|_TIF_USER_WORK_MASK|_TIF_PERSYSCALL_MASK)
	bne-	syscall_exit_work

Which is:

syscall_exit_work:
#ifdef CONFIG_PPC_BOOK3S
	mtmsrd	r10,1		/* Restore RI */
#endif

So that appears to still work. But it's super fragile.

What I'd like to do is drop that optimisation of clearing RI early with
EE. That would avoid us needing to restore RI in syscall_exit_work and
before restore_math (and reclearing it after).

But I guess that will hurt performance too much :/

cheers