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

On Tue, 20 Sep 2016 14:25:48 +1000
Michael Ellerman [off-list ref] wrote:

Nicholas Piggin [off-list ref] writes:

quoted

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 ...

quoted

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.

quoted

@@ -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.
	 */

quoted

 #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.

Agreed. We'll go with the idea you mentioned offline to just load r10 again
here to avoid the long dependency -- it's not going to be a measurable cost.

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 :/

Yes that's something to look into. Newer cores, more kernel code using fp
registers. I'll look into it.

Thanks,
Nick