linux-kernel - Re: [RFC PATCH v2 12/13] powerpc/kernel: Do not inconditionally save non volatile registers on system call

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Message-Id: <1586218971.lolwg4f0lh.astroid@bobo.none>
Date:   Tue, 07 Apr 2020 10:26:58 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@....fr>,
        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: [RFC PATCH v2 12/13] powerpc/kernel: Do not inconditionally save
 non volatile registers on system call

Christophe Leroy's on April 7, 2020 4:18 am:
> 
> 
> Le 06/04/2020 à 03:25, Nicholas Piggin a écrit :
>> Christophe Leroy's on April 6, 2020 3:44 am:
>>> Before : 347 cycles on null_syscall
>>> After  : 327 cycles on null_syscall
>> 
>> The problem I had doing this is that signal delivery wnats full regs,
>> and you don't know if you have a signal pending ahead of time if you
>> have interrupts enabled.
>> 
>> I began to try bailing out back to asm to save nvgprs and call again.
>> I think that can be made to work, but it is more complication in asm,
>> and I soon found that 64s CPUs don't care about NVGPRs too much so it's
>> nice to get rid of the !fullregs state.
> 
> I tried a new way in v3, please have a look. I split 
> syscall_exit_prepare() in 3 parts and the result is unexpected: it is 
> better than before the series (307 cycles now versus 311 cycles with 
> full ASM syscall entry/exit).

Great! Well I don't really see a problem with how you changed the C code 
around. I'll have to look at the assembly but I don't think it would 
have caused a problem for 64s.

Thanks,
Nick