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powerpc/64s: Remove MSR_RI optimisation in system_call_exit()

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Currently in system_call_exit() we have an optimisation where we
disable MSR_RI (recoverable interrupt) and MSR_EE (external interrupt
enable) in a single mtmsrd instruction.

Unfortunately this will no longer work with THREAD_INFO_IN_TASK,
because then the load of TI_FLAGS might fault and faulting with MSR_RI
clear is treated as an unrecoverable exception which leads to a
panic().

So change the code to only clear MSR_EE prior to loading TI_FLAGS,
leaving the clear of MSR_RI until later. We have some latitude in
where do the clear of MSR_RI. A bit of experimentation has shown that
this location gives the least slow down.

This still causes a noticeable slow down in our null_syscall
performance. On a Power9 DD2.2:

  Before        After         Delta     Delta %
  955 cycles    999 cycles    -44	-4.6%

On the plus side this does simplify the code somewhat, because we
don't have to reenable MSR_RI on the restore_math() or
syscall_exit_work() paths which was necessitated previously by the
optimisation.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
This commit is contained in:
Michael Ellerman 2018-11-29 17:42:24 +11:00
parent fb0b0a73b2
commit e7fda7e569
1 changed files with 14 additions and 20 deletions
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34
arch/powerpc/kernel/entry_64.S
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@ -236,18 +236,14 @@ system_call_exit:
/*
* Disable interrupts so current_thread_info()->flags can't change,
* and so that we don't get interrupted after loading SRR0/1.
*
* Leave MSR_RI enabled for now, because with THREAD_INFO_IN_TASK we
* could fault on the load of the TI_FLAGS below.
*/
#ifdef CONFIG_PPC_BOOK3E
wrteei 0
#else
/*
* 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 r11,0
li r11,MSR_RI
mtmsrd r11,1
#endif /* CONFIG_PPC_BOOK3E */
@ -263,15 +259,7 @@ system_call_exit:
bne 3f
#endif
2: addi r3,r1,STACK_FRAME_OVERHEAD
#ifdef CONFIG_PPC_BOOK3S
li r10,MSR_RI
mtmsrd r10,1 /* Restore RI */
#endif
bl restore_math
#ifdef CONFIG_PPC_BOOK3S
li r11,0
mtmsrd r11,1
#endif
ld r8,_MSR(r1)
ld r3,RESULT(r1)
li r11,-MAX_ERRNO
@ -287,6 +275,16 @@ END_FTR_SECTION_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS)
andi. r6,r8,MSR_PR
ld r4,_LINK(r1)
#ifdef CONFIG_PPC_BOOK3S
/*
* Clear MSR_RI, MSR_EE is already and remains disabled. We could do
* this later, but testing shows that doing it here causes less slow
* down than doing it closer to the rfid.
*/
li r11,0
mtmsrd r11,1
#endif
beq- 1f
ACCOUNT_CPU_USER_EXIT(r13, r11, r12)
@ -363,10 +361,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
b .Lsyscall_exit
.Lsyscall_exit_work:
#ifdef CONFIG_PPC_BOOK3S
li r10,MSR_RI
mtmsrd r10,1 /* Restore RI */
#endif
/* If TIF_RESTOREALL is set, don't scribble on either r3 or ccr.
If TIF_NOERROR is set, just save r3 as it is. */