* [PATCH] powerpc/64s: Remove MSR_RI optimisation in system_call_exit()
@ 2019-01-17 11:35 Michael Ellerman
2019-01-23 8:31 ` Nicholas Piggin
2019-02-26 3:27 ` Michael Ellerman
0 siblings, 2 replies; 4+ messages in thread
From: Michael Ellerman @ 2019-01-17 11:35 UTC (permalink / raw)
To: linuxppc-dev; +Cc: npiggin
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.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
arch/powerpc/kernel/entry_64.S | 34 ++++++++++++++--------------------
1 file changed, 14 insertions(+), 20 deletions(-)
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 83bddacd7a17..b1aea6680a9d 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -236,18 +236,14 @@ system_call: /* label this so stack traces look sane */
/*
* 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: /* label this so stack traces look sane */
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. */
--
2.20.1
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [PATCH] powerpc/64s: Remove MSR_RI optimisation in system_call_exit()
2019-01-17 11:35 [PATCH] powerpc/64s: Remove MSR_RI optimisation in system_call_exit() Michael Ellerman
@ 2019-01-23 8:31 ` Nicholas Piggin
2019-02-04 12:15 ` Michael Ellerman
2019-02-26 3:27 ` Michael Ellerman
1 sibling, 1 reply; 4+ messages in thread
From: Nicholas Piggin @ 2019-01-23 8:31 UTC (permalink / raw)
To: linuxppc-dev, Michael Ellerman
Michael Ellerman's on January 17, 2019 9:35 pm:
> 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.
>
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
But only because spectre and meltdown broke my spirit.
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH] powerpc/64s: Remove MSR_RI optimisation in system_call_exit()
2019-01-23 8:31 ` Nicholas Piggin
@ 2019-02-04 12:15 ` Michael Ellerman
0 siblings, 0 replies; 4+ messages in thread
From: Michael Ellerman @ 2019-02-04 12:15 UTC (permalink / raw)
To: Nicholas Piggin, linuxppc-dev
Nicholas Piggin <npiggin@gmail.com> writes:
> Michael Ellerman's on January 17, 2019 9:35 pm:
>> 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.
>>
>> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
>
> Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
>
> But only because spectre and meltdown broke my spirit.
😭😭😭😭😭😭😭😭😭😭
Thanks for reviewing it anyway.
cheers
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: powerpc/64s: Remove MSR_RI optimisation in system_call_exit()
2019-01-17 11:35 [PATCH] powerpc/64s: Remove MSR_RI optimisation in system_call_exit() Michael Ellerman
2019-01-23 8:31 ` Nicholas Piggin
@ 2019-02-26 3:27 ` Michael Ellerman
1 sibling, 0 replies; 4+ messages in thread
From: Michael Ellerman @ 2019-02-26 3:27 UTC (permalink / raw)
To: Michael Ellerman, linuxppc-dev; +Cc: npiggin
On Thu, 2019-01-17 at 11:35:10 UTC, Michael Ellerman wrote:
> 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.
>
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Applied to powerpc next.
https://git.kernel.org/powerpc/c/e7fda7e569e1776d4dccbcef52d34882
cheers
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2019-02-26 3:29 UTC | newest]
Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-01-17 11:35 [PATCH] powerpc/64s: Remove MSR_RI optimisation in system_call_exit() Michael Ellerman
2019-01-23 8:31 ` Nicholas Piggin
2019-02-04 12:15 ` Michael Ellerman
2019-02-26 3:27 ` Michael Ellerman
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).