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

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Date:   Tue, 9 Feb 2021 15:31:02 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Nicholas Piggin <npiggin@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        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



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.

Christophe


> 
> 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
>>
>>