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 16:56:01 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        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()

Christophe Leroy <christophe.leroy@...roup.eu> writes:
> 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 ?

Yeah I guess.

I'm not sure if we'll want to keep cpu_has_msr_ri() if we have a
CONFIG_PPC_MSR_RI, but that's a pretty minor detail.

So yeah I'll take patch 2 as well.

cheers