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From: Bruno Piazera Larsen <bruno.larsen@eldorado.org.br>
To: Richard Henderson <richard.henderson@linaro.org>, qemu-devel@nongnu.org
Cc: farosas@linux.ibm.com, luis.pires@eldorado.org.br,
	Greg Kurz <groug@kaod.org>,
	lucas.araujo@eldorado.org.br, fernando.valle@eldorado.org.br,
	qemu-ppc@nongnu.org, matheus.ferst@eldorado.org.br,
	david@gibson.dropbear.id.au
Subject: Re: [RFC PATCH] target/ppc: fix address translation bug for hash table mmus
Date: Tue, 8 Jun 2021 15:39:25 -0300	[thread overview]
Message-ID: <524da033-7d6a-5720-026a-04fbc01e40c3@eldorado.org.br> (raw)
In-Reply-To: <258682ee-0ae6-cf59-e7bf-42879abcde5b@eldorado.org.br>

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On 08/06/2021 13:37, Bruno Piazera Larsen wrote:
>
>
> On 08/06/2021 12:35, Richard Henderson wrote:
>> On 6/8/21 7:39 AM, Bruno Piazera Larsen wrote:
>>>> That's odd.  We already have more arguments than the number of 
>>>> argument registers...  A 5x slowdown is distinctly odd.
>>> I did some more digging and the problem is not with 
>>> ppc_radix64_check_prot, the problem is ppc_radix64_xlate, which 
>>> currently has 7 arguments and we're increasing to 8. 7 feels like 
>>> the correct number, but I couldn't find docs supporting it, so I 
>>> could be wrong.
>>
>> According to tcg/ppc/tcg-target.c.inc, there are 8 argument registers 
>> for ppc hosts.  But now I see you didn't actually say on which host 
>> you observed the problem...  It's 6 argument registers for x86_64 host.
>
> Oh, yes, sorry. I'm experiencing it in a POWER9 machine (ppc64le 
> architecture). According to tcg this shouldn't be the issue, then, so 
> idk if that's the real reason or not. All I know is that as soon as 
> gcc can't optimize an argument away it happens (fprintf in 
> radix64_xlate, using one of the mmuidx_* functions, defining those as 
> macros).
>
> I'll test it in my x86_64 machine and see if such a slowdown happens. 
> It's not conclusive evidence, but the function is too complex for me 
> to follow the disassembly if I can avoid it...
>
Test has been done: Slow down also happens on the x86_64 machine (but 
without change its already 360s, so idk if the slowdown is that 
dramatic), so it's _probably_ not going over the argument register 
count. I have no clue what could be. Still working on the struct version 
to see if anything changes.

-- 
Bruno Piazera Larsen
Instituto de Pesquisas ELDORADO 
<https://www.eldorado.org.br/?utm_campaign=assinatura_de_e-mail&utm_medium=email&utm_source=RD+Station>
Departamento Computação Embarcada
Analista de Software Trainee
Aviso Legal - Disclaimer <https://www.eldorado.org.br/disclaimer.html>

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      reply	other threads:[~2021-06-08 18:42 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-02 19:18 Bruno Larsen (billionai)
2021-06-02 19:26 ` Richard Henderson
2021-06-02 19:58   ` Bruno Piazera Larsen
2021-06-02 22:19     ` Richard Henderson
2021-06-07 19:29       ` Bruno Piazera Larsen
2021-06-07 21:06         ` Richard Henderson
2021-06-08 14:39           ` Bruno Piazera Larsen
2021-06-08 15:35             ` Richard Henderson
2021-06-08 16:37               ` Bruno Piazera Larsen
2021-06-08 18:39                 ` Bruno Piazera Larsen [this message]

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