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SubjectRe: [RFC PATCH] powerpc/6xx: Don't set back MSR_RI before reenabling MMU
Date
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.

cheers

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