linux-kernel - Re: [PATCH v5 16/22] powerpc/syscall: Avoid stack frame in likely part of system_call

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Date:   Wed, 10 Feb 2021 11:56:28 +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 16/22] powerpc/syscall: Avoid stack frame in likely
 part of system_call_exception()

Excerpts from Christophe Leroy's message of February 10, 2021 2:13 am:
> 
> 
> Le 09/02/2021 à 02:55, Nicholas Piggin a écrit :
>> Excerpts from Christophe Leroy's message of February 9, 2021 1:10 am:
>>> When r3 is not modified, reload it from regs->orig_r3 to free
>>> volatile registers. This avoids a stack frame for the likely part
>>> of system_call_exception()
>> 
>> This doesn't on my 64s build, but it does reduce one non volatile
>> register save/restore. With quite a bit more register pressure
>> reduction 64s can avoid the stack frame as well.
> 
> The stack frame is not due to the registers because on PPC64 you have the redzone that you don't 
> have on PPC32.
> 
> As far as I can see, this is due to a call to .arch_local_irq_restore().
> 
> On ppc32 arch_local_irq_restore() is just a write to MSR.

Oh you're right there. We can actually inline fast paths of that I have 
a patch somewhere, but not sure if it's worthwhile.

>> It's a cool trick but quite code and compiler specific so I don't know
>> how worthwhile it is to keep considering we're calling out into random
>> kernel C code after this.
>> 
>> Maybe just keep it PPC32 specific for the moment, will have to do more
>> tuning for 64 and we have other stuff to do there first.
>> 
>> If you are happy to make it 32-bit only then
> 
> I think we can leave without this, that's only one or two cycles won.

Okay for this round let's drop it for now.

Thanks,
Nick