All the mail mirrored from lore.kernel.org
 help / color / mirror / Atom feed
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: Mon, 7 Jun 2021 16:29:15 -0300	[thread overview]
Message-ID: <c79e73b0-f989-575b-0982-672fc9b289eb@eldorado.org.br> (raw)
In-Reply-To: <b689bdd0-4d75-7c4e-189e-922738208dc0@linaro.org>

[-- Attachment #1: Type: text/plain, Size: 4275 bytes --]


On 02/06/2021 19:19, Richard Henderson wrote:
> On 6/2/21 12:58 PM, Bruno Piazera Larsen wrote:
>>> For the use from ppc_cpu_get_phys_page_debug, you'd pass in 
>>> cpu_mmu_index(env, false).
>>
>> ppc_cpu_get_phys_page_debug has 2 calls to ppc_xlate, one using the 
>> data MMU, the other using the instruction MMU. I'm guessing I should 
>> pass both, right?
>
> Yes.
>
>> But here we have another bit that confuses me: cpu_mmu_index returns 
>> 0 if in user mode, or uses the information stored in env to get it, 
>> so I don't see how that would be different from getting directly. 
>> Unless the point is to have ppc_*_xlate be generic and pc_*_debug 
>> knows the info in env is correct. Is that it?
>
> The issue is that
>
> (1) ppc_*_xlate should perform the lookup requested, and mmu_idx
>     does not *necessarily* correspond to the current contents of
>     env->msr et al.  See (2).
>
> (2) There is a secondary call to ppc_radix64_partition_scoped_xlate
>     for which the second stage page table should be read
>     with hypervisor permissions, and not the permissions of the
>     original memory access.
>
>     Note that ppc_radix64_check_prot checks msr_pr directly.
>
>     Thus the second stage lookup should use mmu_idx = 5
>     (HV kernel virtual mode).  If I understand things correctly...
>
>>
>>>
>>>
>>>> +    const short HV = 1, IR = 2, DR = 3;
>>>> +    bool MSR[3];
>>>> +    MSR[HV] = dmmu_idx & 2,
>>>> +    MSR[IR] = immu_idx & 4,
>>>> +    MSR[DR] = dmmu_idx & 4;
>>>
>>> There's no point in the array.  Just use three different scalars 
>>> (real_mode, hv, and pr (note that pr is the major portion of the bug 
>>> as reported)). Additionally, you'll not be distinguishing immu_idx 
>>> and dmmu_idx, but using the single idx that's given.
>>
>> Ah, yeah, that's the "more complex than necessary, but it was easy 
>> for me to read" part. Scalars are a good solution. In this function 
>> in specific, PR doesn't actually show up anywhere, so I would 
>> actually only need 2. Anyway, will start working on this.
>
> Oh, I'll note that your constants above are wrong.  I think that you 
> should have some common routines in (mmu-)internal.h:
>
> /*
>  * These correspond to the mmu_idx values computed in
>  * hreg_compute_hflags_value.  See the tables therein.
>  */
> static inline bool mmuidx_pr(int idx) { return idx & 1; }
> static inline bool mmuidx_real(int idx) { return idx & 2; }
> static inline bool mmuidx_hv(int idx) { return idx & 4; }
>
> because you'll want to use these past mmu-radix64.c.
>
> Then you also have a single place to adjust if the mmu_idx are 
> reordered at a later date.
>
>
> r~

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). 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;

That way we continue to use the env as the way to send the variable and 
avoid what I think is a problem with the compiler's optimization.

Would this be acceptable (with proper documentation in the form of 
comments, ofc) or do we have to find a better solution that doesn't 
touch the values of env? I personally don't like it, but I couldn't find 
a better solution. If you're fine with it, we can use it, otherwise I'll 
keep looking.

-- 
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 If
<https://www.eldorado.org.br/disclaimer.html>

[-- Attachment #2: Type: text/html, Size: 6172 bytes --]

  reply	other threads:[~2021-06-07 19:32 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-02 19:18 [RFC PATCH] target/ppc: fix address translation bug for hash table mmus 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 [this message]
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=c79e73b0-f989-575b-0982-672fc9b289eb@eldorado.org.br \
    --to=bruno.larsen@eldorado.org.br \
    --cc=david@gibson.dropbear.id.au \
    --cc=farosas@linux.ibm.com \
    --cc=fernando.valle@eldorado.org.br \
    --cc=groug@kaod.org \
    --cc=lucas.araujo@eldorado.org.br \
    --cc=luis.pires@eldorado.org.br \
    --cc=matheus.ferst@eldorado.org.br \
    --cc=qemu-devel@nongnu.org \
    --cc=qemu-ppc@nongnu.org \
    --cc=richard.henderson@linaro.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.