Commit e7fda7e5 authored by Michael Ellerman's avatar Michael Ellerman
Browse files

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



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: default avatarNicholas Piggin <npiggin@gmail.com>
Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
parent fb0b0a73

arch/powerpc/kernel/entry_64.S

+14 −20
Original line number Diff line number Diff line
@@ -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. */

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