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

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Message-Id: <1628067212.yli25q0lwj.astroid@bobo.none>
Date:   Wed, 04 Aug 2021 18:53:47 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc: Remove MSR_PR check in
 interrupt_exit_{user/kernel}_prepare()

Excerpts from Christophe Leroy's message of August 4, 2021 6:37 pm:
> 
> 
> Le 04/08/2021 à 10:08, Nicholas Piggin a écrit :
>> Excerpts from Christophe Leroy's message of August 4, 2021 3:27 pm:
>>> 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>
>> 
>> Probably okay to do now things are ironing out.
>> 
>> Unless we want to make a new define for interrupt handler debug and put
>> a bunch of these asserts under it. There's quite a lot more here, and
>> in asm/interrupt.h, etc.
> 
> But that one is so trivial that I'm not sure there is any point in keeping it even as a kind of 
> additional DEBUG level, unless you want those BUG_ONs because you don't trust the compiler.

Fair point.

Acked-by: Nicholas Piggin <npiggin@...il.com>