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

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Message-ID: <cd5f54fd-fbf4-e471-9971-1e8c86755754@csgroup.eu>
Date:   Wed, 4 Aug 2021 10:37:08 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Nicholas Piggin <npiggin@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        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()



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.

Christophe

> 
> Thanks,
> Nick
> 
>> ---
>>   arch/powerpc/kernel/interrupt.c | 2 --
>>   1 file changed, 2 deletions(-)
>>
>> diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
>> index 21bbd615ca41..f26caf911ab5 100644
>> --- a/arch/powerpc/kernel/interrupt.c
>> +++ b/arch/powerpc/kernel/interrupt.c
>> @@ -465,7 +465,6 @@ notrace unsigned long interrupt_exit_user_prepare(struct pt_regs *regs)
>>   
>>   	if (!IS_ENABLED(CONFIG_BOOKE) && !IS_ENABLED(CONFIG_40x))
>>   		BUG_ON(!(regs->msr & MSR_RI));
>> -	BUG_ON(!(regs->msr & MSR_PR));
>>   	BUG_ON(arch_irq_disabled_regs(regs));
>>   	CT_WARN_ON(ct_state() == CONTEXT_USER);
>>   
>> @@ -499,7 +498,6 @@ notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs)
>>   	if (!IS_ENABLED(CONFIG_BOOKE) && !IS_ENABLED(CONFIG_40x) &&
>>   	    unlikely(!(regs->msr & MSR_RI)))
>>   		unrecoverable_exception(regs);
>> -	BUG_ON(regs->msr & MSR_PR);
>>   	/*
>>   	 * CT_WARN_ON comes here via program_check_exception,
>>   	 * so avoid recursion.
>> -- 
>> 2.25.0
>>
>>