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SubjectRe: [RFC PATCH] powerpc/6xx: Don't set back MSR_RI before reenabling MMU
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Le 01/02/2019 à 12:10, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@c-s.fr> writes:
>
>> By delaying the setting of MSR_RI, a 1% improvment is optained on
>> null_syscall selftest on an mpc8321.
>>
>> Without this patch:
>>
>> root@vgoippro:~# ./null_syscall
>> 1134.33 ns 378.11 cycles
>>
>> With this patch:
>>
>> root@vgoippro:~# ./null_syscall
>> 1121.85 ns 373.95 cycles
>>
>> The drawback is that a machine check during that period
>> would be unrecoverable, but as only main memory is accessed
>> during that period, it shouldn't be a concern.
>
> On 64-bit server CPUs accessing main memory can cause a UE
> (Uncorrectable Error) which can trigger a machine check.
>
> So it may still be a concern, it depends how paranoid you are.
>
>> diff --git a/arch/powerpc/kernel/head_32.S b/arch/powerpc/kernel/head_32.S
>> index 146385b1c2da..ea28a6ab56ec 100644
>> --- a/arch/powerpc/kernel/head_32.S
>> +++ b/arch/powerpc/kernel/head_32.S
>> @@ -282,8 +282,6 @@ __secondary_hold_acknowledge:
>> stw r1,GPR1(r11); \
>> stw r1,0(r11); \
>> tovirt(r1,r11); /* set new kernel sp */ \
>> - li r10,MSR_KERNEL & ~(MSR_IR|MSR_DR); /* can take exceptions */ \
>> - MTMSRD(r10); /* (except for mach check in rtas) */ \
>> stw r0,GPR0(r11); \
>> lis r10,STACK_FRAME_REGS_MARKER@ha; /* exception frame marker */ \
>> addi r10,r10,STACK_FRAME_REGS_MARKER@l; \
>
> Where does RI get enabled? I don't see it anywhere obvious.

MSR_RI is part of MSR_KERNEL, it gets then enabled when reenabling MMU
when calling the exception handler.

#define EXC_XFER_TEMPLATE(n, hdlr, trap, copyee, tfer, ret) \
li r10,trap; \
stw r10,_TRAP(r11); \
li r10,MSR_KERNEL; \
copyee(r10, r9); \
bl tfer; \
i##n: \
.long hdlr; \
.long ret

where tfer = transfer_to_handler.

In transfer_to_handler (kernel/entry_32.S) you have:

transfer_to_handler_cont:
3:
mflr r9
lwz r11,0(r9) /* virtual address of handler */
lwz r9,4(r9) /* where to go when done */
[...]
mtspr SPRN_SRR0,r11
mtspr SPRN_SRR1,r10
mtlr r9
SYNC
RFI /* jump to handler, enable MMU */

So MSR_RI is restored above as r10 contains MSR_KERNEL [ | MSR_EE ]

Christophe



>
> cheers
>

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