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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Christophe Leroy <christophe.leroy@c-s.fr>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [RFC PATCH] powerpc/6xx: Don't set back MSR_RI before reenabling MMU
In-Reply-To: <9f9dd859d571e324c7412ed9db9da8cfba678257.1548956511.git.christophe.leroy@c-s.fr>
References: <9f9dd859d571e324c7412ed9db9da8cfba678257.1548956511.git.christophe.leroy@c-s.fr>
Date:   Fri, 01 Feb 2019 22:10:30 +1100
Message-ID: <87lg2zojbt.fsf@concordia.ellerman.id.au>
MIME-Version: 1.0
Content-Type: text/plain
Sender: linux-kernel-owner@vger.kernel.org
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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