linux-kernel - Re: [RFC WIP PATCH] powerpc/32: system call implement entry/exit logic in C

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Date:   Sun, 5 Apr 2020 20:53:00 +0200
From:   Christophe Leroy <christophe.leroy@....fr>
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: [RFC WIP PATCH] powerpc/32: system call implement entry/exit
 logic in C



Le 03/04/2020 à 09:33, Nicholas Piggin a écrit :
> Christophe Leroy's on April 1, 2020 9:48 pm:
>>
>>
>> Le 31/03/2020 à 17:22, Christophe Leroy a écrit :
>>> That's first try to port PPC64 syscall entry/exit logic in C to PPC32.
>>> I've do the minimum to get it work. I have not reworked calls
>>> to sys_fork() and friends for instance.
>>>
>>> For the time being, it seems to work more or less but:
>>> - ping reports EINVAL on recvfrom
>>> - strace shows NULL instead of strings in call like open() for instance.
>>
>> For the two above problems, that's because system_call_exception()
>> doesn't set orig_gpr3 whereas DoSycall() does in entry_32.S . Is that
>> only done on PPC32 ?
>>
>> With the following line at the begining of system_call_exception(), it
>> works perfectly:
>>
>> 	regs->orig_gpr3 = r3;
> 
> Oh great, nice work. We should be able to make some simple helpers or
> move some things a bit to reduce the amount of ifdefs in the C code.
> It doesn't look too bad though.
> 
>> I will now focus on performance to see if we can do something about it.
> 
> What's the performance difference between current asm code just with
> always saving NVGPRS vs C?

Done new measurement and sent a series. lower values now because the 
time accounting was done twice as it was still in the ASM part.

Before the series, 311 cycles for a null_syscall
If adding SAVE_NVGPRS to the entry macro, 335 cycles

First patch: 353 cycles ie +13,5%

After a few changes, including conditional saving of non volatile 
registers, I get 325 cycles that is only +4,5%. I thing that's acceptable.

Do you see a problem with still saving non volatile registers only when 
required ?

Christophe