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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>, npiggin@gmail.com
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v3 1/3] powerpc: Remove MSR_PR check in
 interrupt_exit_{user/kernel}_prepare()
In-Reply-To: <385ead49ccb66a259b25fee3eebf0bd4094068f3.1629707037.git.christophe.leroy@csgroup.eu>
References: <385ead49ccb66a259b25fee3eebf0bd4094068f3.1629707037.git.christophe.leroy@csgroup.eu>
Date:   Wed, 25 Aug 2021 15:27:22 +1000
Message-ID: <87zgt6aybp.fsf@mpe.ellerman.id.au>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Christophe Leroy <christophe.leroy@csgroup.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@csgroup.eu>
> Acked-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>  arch/powerpc/kernel/interrupt.c | 2 --
>  1 file changed, 2 deletions(-)

I'll pick this one up independent of the other two patches.

cheers