linux-kernel - Re: [PATCH v5 18/22] powerpc/syscall: Remove FULL_REGS verification in system_call

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Date:   Tue, 09 Feb 2021 12:02:30 +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>, msuchanek@...e.de,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v5 18/22] powerpc/syscall: Remove FULL_REGS verification
 in system_call_exception

Excerpts from Christophe Leroy's message of February 9, 2021 1:10 am:
> For book3s/64, FULL_REGS() is 'true' at all time, so the test voids.
> For others, non volatile registers are saved inconditionally.
> 
> So the verification is pointless.
> 
> Should one fail to do it, it would anyway be caught by the
> CHECK_FULL_REGS() in copy_thread() as we have removed the
> special versions ppc_fork() and friends.
> 
> null_syscall benchmark reduction 4 cycles (332 => 328 cycles)

I wonder if we rather make a CONFIG option for a bunch of these simpler
debug checks here (and also in interrupt exit, wrappers, etc) rather
than remove them entirely.

Thanks,
Nick

> 
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> ---
>  arch/powerpc/kernel/interrupt.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
> index 8fafca727b8b..55e1aa18cdb9 100644
> --- a/arch/powerpc/kernel/interrupt.c
> +++ b/arch/powerpc/kernel/interrupt.c
> @@ -42,7 +42,6 @@ notrace long system_call_exception(long r3, long r4, long r5,
>  	if (!IS_ENABLED(CONFIG_BOOKE) && !IS_ENABLED(CONFIG_40x))
>  		BUG_ON(!(regs->msr & MSR_RI));
>  	BUG_ON(!(regs->msr & MSR_PR));
> -	BUG_ON(!FULL_REGS(regs));
>  	BUG_ON(arch_irq_disabled_regs(regs));
>  
>  #ifdef CONFIG_PPC_PKEY
> -- 
> 2.25.0
> 
>