Re: [PATCH] powerpc: Remove MSR_PR check in interrupt_exit_{user/kernel}_prepare()

[Christophe Leroy]

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@xxxxxxxxxx>
>
> 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