linux-kernel - Re: [RFC PATCH] powerpc/6xx: Don't set back MSR

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Message-ID: <17b00097-a807-e9b9-3dce-198df4a3153f@c-s.fr>
Date:   Fri, 1 Feb 2019 12:51:54 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH] powerpc/6xx: Don't set back MSR_RI before reenabling
 MMU



Le 01/02/2019 à 12:10, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@....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@...ippro:~# ./null_syscall
>>     1134.33 ns     378.11 cycles
>>
>> With this patch:
>>
>> root@...ippro:~# ./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
>