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

[Michael Ellerman]

Christophe Leroy <christophe.leroy at 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 at vgoippro:~# ./null_syscall
>    1134.33 ns     378.11 cycles
>
> With this patch:
>
> root at 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 at ha; /* exception frame marker */ \
>  	addi	r10,r10,STACK_FRAME_REGS_MARKER at l; \

Where does RI get enabled? I don't see it anywhere obvious.

cheers