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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 11:39:24 -0300	[thread overview]
Message-ID: <b5834a1f-afaa-a36a-11d6-35a197ad74bc@eldorado.org.br> (raw)
In-Reply-To: <7198ccf1-f2db-2e39-3778-4083b5d7fa45@linaro.org>

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On 07/06/2021 18:06, Richard Henderson wrote:
> On 6/7/21 12:29 PM, Bruno Piazera Larsen wrote:
>> I just tried sending mmu_idx all the way down, but I ran into a very 
>> weird bug of gcc. If we have to add one more parameter that GCC can't 
>> just optimize away we get at least a slow down of 5x for the first 
>> test of check-acceptance (could be more, but the test times out after 
>> 900 seconds, so I'm not sure).
>
> 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.
>
>
>> One way that I managed to get around that is saving the current MSR, 
>> setting it to 5, and restoring after the xlate call. The code ended 
>> up something like:
>>
>> int new_idx = (5<<HFLAGS_IMMU_IDX) | (5<<HFLAGS_DMMU_IDX);
>> int clr = (7<<HFLAGS_IMMU_IDX) | (7<<HFLAGS_DMMU_IDX);
>> int old_idx = env->msr & clr;
>> clr = ~clr;
>> /* set new msr so we don't need to send the mmu_idx */
>> env->msr = (env->msr & clr) | new_idx;
>> ret = ppc_radix64_partition_scoped_xlate(...);
>> /* restore old mmu_idx */
>> env->msr = (env->msr & clr) | old_idx;
>
> No, this is silly.
>
> We need to do one of two things:
>   - make sure everything is inlined,
>   - reduce the number of arguments.
>
> We're currently passing in 9 arguments, which really is too many 
> already.  We should be using something akin to mmu_ctx_t, but probably 
> specific to radix64 without the random stuff collected for random 
> other mmu models.

That means we'd have to define radix_ctx_t (or however we call it) in 
radix64.h, setup the struct on ppc_xlate, then pass it to ppc_radix64_xlate.

 From looking at the code, it seems the most useful bits to put in the 
struct are: eaddr, g_addr, h_addr, {h,g}_prot, {g,h}_page_size, mmu_idx 
and guest_visible. They all seem reasonable to me, but I might be 
missing something again.

>
>
> r~
Another question: I know hash mmus don't have this problem, but since 
ppc_jumbo_xlate also uses mmu_idx, should we make those xlate user 
mmu_idxs as well? I tested and it doesn't make a time difference.
-- 
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 14:41 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 [this message]
2021-06-08 15:35             ` Richard Henderson
2021-06-08 16:37               ` Bruno Piazera Larsen
2021-06-08 18:39                 ` Bruno Piazera Larsen

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