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

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Date:   Wed, 10 Feb 2021 11:57:40 +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 10, 2021 12:31 am:
> 
> 
> Le 09/02/2021 à 03:02, Nicholas Piggin a écrit :
>> 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.
> 
> We can drop this patch if you prefer. Anyway, like book3s/64, once ppc32 also do interrupt 
> entry/exit in C, FULL_REGS() will already return true.

Sure let's do that.

Thanks,
Nick