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

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Message-Id: <1628064412.48kzr1eula.astroid@bobo.none>
Date:   Wed, 04 Aug 2021 18:08:16 +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 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.

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
> 
>