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Date:   Tue, 09 Feb 2021 12:02:30 +1000
From:   Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH v5 18/22] powerpc/syscall: Remove FULL_REGS verification
 in system_call_exception
To:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Michael Ellerman <mpe@ellerman.id.au>, msuchanek@suse.de,
        Paul Mackerras <paulus@samba.org>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
References: <cover.1612796617.git.christophe.leroy@csgroup.eu>
        <6bef4d9ba0cba50160d13e344ee4627ebdf801dc.1612796617.git.christophe.leroy@csgroup.eu>
In-Reply-To: <6bef4d9ba0cba50160d13e344ee4627ebdf801dc.1612796617.git.christophe.leroy@csgroup.eu>
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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.
>=20
> So the verification is pointless.
>=20
> 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.
>=20
> null_syscall benchmark reduction 4 cycles (332 =3D> 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

>=20
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
>  arch/powerpc/kernel/interrupt.c | 1 -
>  1 file changed, 1 deletion(-)
>=20
> diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interr=
upt.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, lo=
ng 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));
> =20
>  #ifdef CONFIG_PPC_PKEY
> --=20
> 2.25.0
>=20
>=20