linux-kernel - Re: [PATCH v3 1/3] powerpc: Remove MSR_PR check in interrupt_exit_{user/kernel}

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Date:   Wed, 25 Aug 2021 07:45:02 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, npiggin@...il.com
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v3 1/3] powerpc: Remove MSR_PR check in
 interrupt_exit_{user/kernel}_prepare()



Le 25/08/2021 à 07:27, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@...roup.eu> writes:
>> In those hot functions that are called at every interrupt, any saved
>> cycle is worth it.
>>
>> interrupt_exit_user_prepare() and interrupt_exit_kernel_prepare() are
>> called from three places:
>> - From entry_32.S
>> - From interrupt_64.S
>> - From interrupt_exit_user_restart() and interrupt_exit_kernel_restart()
>>
>> In entry_32.S, there are inambiguously called based on MSR_PR:
>>
>> 	interrupt_return:
>> 		lwz	r4,_MSR(r1)
>> 		addi	r3,r1,STACK_FRAME_OVERHEAD
>> 		andi.	r0,r4,MSR_PR
>> 		beq	.Lkernel_interrupt_return
>> 		bl	interrupt_exit_user_prepare
>> 	...
>> 	.Lkernel_interrupt_return:
>> 		bl	interrupt_exit_kernel_prepare
>>
>> In interrupt_64.S, that's similar:
>>
>> 	interrupt_return_\srr\():
>> 		ld	r4,_MSR(r1)
>> 		andi.	r0,r4,MSR_PR
>> 		beq	interrupt_return_\srr\()_kernel
>> 	interrupt_return_\srr\()_user: /* make backtraces match the _kernel variant */
>> 		addi	r3,r1,STACK_FRAME_OVERHEAD
>> 		bl	interrupt_exit_user_prepare
>> 	...
>> 	interrupt_return_\srr\()_kernel:
>> 		addi	r3,r1,STACK_FRAME_OVERHEAD
>> 		bl	interrupt_exit_kernel_prepare
>>
>> In interrupt_exit_user_restart() and interrupt_exit_kernel_restart(),
>> MSR_PR is verified respectively by BUG_ON(!user_mode(regs)) and
>> BUG_ON(user_mode(regs)) prior to calling interrupt_exit_user_prepare()
>> and interrupt_exit_kernel_prepare().
>>
>> The verification in interrupt_exit_user_prepare() and
>> interrupt_exit_kernel_prepare() are therefore useless and can be removed.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>> Acked-by: Nicholas Piggin <npiggin@...il.com>
>> ---
>>   arch/powerpc/kernel/interrupt.c | 2 --
>>   1 file changed, 2 deletions(-)
> 
> I'll pick this one up independent of the other two patches.
> 

Second patch should be ok as well, no ?

Christophe