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From: Richard Henderson <rth@twiddle.net>
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Date: Thu, 27 Apr 2017 00:29:49 +0200
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Subject: Re: [Qemu-devel] [PATCH v3 01/10] tcg-runtime: add lookup_tb_ptr
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To: "Emilio G. Cota" <cota@braap.org>
Cc: qemu-devel@nongnu.org, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Peter Maydell <peter.maydell@linaro.org>, Eduardo Habkost <ehabkost@redhat.com>, Andrzej Zaborowski <balrogg@gmail.com>, Aurelien Jarno <aurelien@aurel32.net>, Alexander Graf <agraf@suse.de>, Stefan Weil <sw@weilnetz.de>, qemu-arm@nongnu.org, alex.bennee@linaro.org, Pranith Kumar <bobby.prani+qemu@gmail.com>

On 04/26/2017 11:56 PM, Emilio G. Cota wrote:
> On Wed, Apr 26, 2017 at 10:40:45 +0200, Richard Henderson wrote:
>> On 04/26/2017 08:23 AM, Emilio G. Cota wrote:
> (snip)
>>> +    cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
>>> +    tb = atomic_rcu_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(addr)]);
>>> +    if (likely(tb && tb->pc == addr && tb->cs_base == cs_base &&
>>> +               tb->flags == flags)) {
>>
>> This comparison is wrong.  It will incorrectly reject a TB for i386 guest
>> when CS_BASE != 0.  You really want
>>
>>    tb = atomic_rcu_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(addr)]);
>>    if (tb) {
>>      cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
>>      if (tb->pc == pc && tb->cs_base == cs_base && tb->flags == flags) {
>>        return tb->tc_ptr;
>>      }
>>    }
>>    return tcg_ctx.code_gen_epilogue;
> 
> wrt the comparison, the only change I notice in your suggested change is
>    tb->pc == pc
> 
> instead of
>    tb->pc == addr
> 
> , which seems innocuous to me (since tb->pc == addr).
> 
> I fail to see how this relates to your "CS_BASE != 0" comment.
> What am I missing?

Recall how you computed vaddr for target/i386:

   addr = pc + cs_base


r~