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Re: [Qemu-devel] [PATCH v3 4/4] tcg: rework tb_invalidated_flag


From: Alex Bennée
Subject: Re: [Qemu-devel] [PATCH v3 4/4] tcg: rework tb_invalidated_flag
Date: Thu, 21 Apr 2016 22:54:41 +0100
User-agent: mu4e 0.9.17; emacs 25.0.92.6

Sergey Fedorov <address@hidden> writes:

> On 21/04/16 19:16, Sergey Fedorov wrote:
>> On 21/04/16 18:55, Alex Bennée wrote:
>>> Sergey Fedorov <address@hidden> writes:
>>>
>>>> On 18/04/16 20:51, Sergey Fedorov wrote:
>>>>> On 18/04/16 20:17, Alex Bennée wrote:
>>>>>> Sergey Fedorov <address@hidden> writes:
>>>>>>> On 18/04/16 17:09, Alex Bennée wrote:
>>>>>>>> Sergey Fedorov <address@hidden> writes:
>>>>>>>>> diff --git a/cpu-exec.c b/cpu-exec.c
>>>>>>> (snip)
>>>>>>>>> @@ -507,14 +510,12 @@ int cpu_exec(CPUState *cpu)
>>>>>>>>>                  }
>>>>>>>>>                  tb_lock();
>>>>>>>>>                  tb = tb_find_fast(cpu);
>>>>>>>>> -                /* Note: we do it here to avoid a gcc bug on Mac OS 
>>>>>>>>> X when
>>>>>>>>> -                   doing it in tb_find_slow */
>>>>>>>> Is this still true? Would it make more sense to push the patching down
>>>>>>>> to the gen_code?
>>>>>>> This comment comes up to the commit:
>>>>>>>
>>>>>>>     commit 1538800276aa7228d74f9d00bf275f54dc9e9b43
>>>>>>>     Author: bellard <address@hidden>
>>>>>>>     Date:   Mon Dec 19 01:42:32 2005 +0000
>>>>>>>
>>>>>>>         workaround for gcc bug on PowerPC
>>>>>>>
>>>>>>>
>>>>>>> It was added more than ten years ago. Anyway, now this code is here not
>>>>>>> because of the bug: we need to reset 'next_tb' which is a local variable
>>>>>>> in cpu_exec(). Personally, I don't think it would be neater to hide it
>>>>>>> into gen_code(). Do you have some thoughts on how we could benefit from
>>>>>>> doing so? BTW, I had a feeling that it may be useful to reorganize
>>>>>>> cpu_exec() a bit, although I don't have a solid idea of how to do this
>>>>>>> so far.
>>>>>> I'm mainly eyeing the tb_lock/unlock which would be nice to push further
>>>>>> down the call chain if we can, especially if the need to lock
>>>>>> tb_find_fast can be removed later on.
>>>>> Yes, it would be nice to possibly have all tb_lock/unlock() calls (or at
>>>>> least their pairs) in the same block. There is a lot to be thought over :)
>>>> It's not so simple because tb_find_fast() is also called in replay mode
>>>> to find a TB for cpu_exec_nocache()... I'm not sure it's worth touching
>>>> it now.
>>> If the locking is pushed into tb_find_fast or further down is this an
>>> issue?
>> We would have to pass 'next_tb' (or 'last_tb' and 'tb_exit' after
>> cleaning it up) if we move TB chaining code to tb_find_fast(). But
>> tb_find_fast() is also called in replay mode to find a TB for
>> cpu_exec_nocache() where we don't bother with TB chaining... Do you
>> think it would be fine to make those changes?
>
> Are you thinking about something like this:
>
> diff --git a/cpu-exec.c b/cpu-exec.c
> index 1d12e8bc2739..07e9ede49193 100644
> --- a/cpu-exec.c
> +++ b/cpu-exec.c
> @@ -320,7 +320,9 @@ found:
>      return tb;
>  }
>
> -static inline TranslationBlock *tb_find_fast(CPUState *cpu)
> +static inline TranslationBlock *tb_find_fast(CPUState *cpu,
> +                                             TranslationBlock **last_tb,
> +                                             int tb_exit)
>  {
>      CPUArchState *env = (CPUArchState *)cpu->env_ptr;
>      TranslationBlock *tb;
> @@ -331,11 +333,27 @@ static inline TranslationBlock
> *tb_find_fast(CPUState *cpu)
>         always be the same before a given translated block
>         is executed. */
>      cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
> +    tb_lock();
>      tb = cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)];
>      if (unlikely(!tb || tb->pc != pc || tb->cs_base != cs_base ||
>                   tb->flags != flags)) {
>          tb = tb_find_slow(cpu, pc, cs_base, flags);
>      }
> +    if (cpu->tb_flushed) {
> +        /* Ensure that no TB jump will be modified as the
> +         * translation buffer has been flushed.
> +         */
> +        *last_tb = NULL;
> +        cpu->tb_flushed = false;
> +    }
> +    /* see if we can patch the calling TB. When the TB
> +       spans two pages, we cannot safely do a direct
> +       jump. */
> +    if (*last_tb != NULL && tb->page_addr[1] == -1
> +            && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
> +        tb_add_jump(*last_tb, tb_exit, tb);
> +    }
> +    tb_unlock();
>      return tb;
>  }
>
> @@ -441,7 +459,8 @@ int cpu_exec(CPUState *cpu)
>              } else if (replay_has_exception()
>                         && cpu->icount_decr.u16.low + cpu->icount_extra
> == 0) {
>                  /* try to cause an exception pending in the log */
> -                cpu_exec_nocache(cpu, 1, tb_find_fast(cpu), true);
> +                last_tb = NULL; /* Avoid chaining TBs */
> +                cpu_exec_nocache(cpu, 1, tb_find_fast(cpu, &last_tb,
> 0), true);
>                  ret = -1;
>                  break;
>  #endif
> @@ -511,23 +530,7 @@ int cpu_exec(CPUState *cpu)
>                      cpu->exception_index = EXCP_INTERRUPT;
>                      cpu_loop_exit(cpu);
>                  }
> -                tb_lock();
> -                tb = tb_find_fast(cpu);
> -                if (cpu->tb_flushed) {
> -                    /* Ensure that no TB jump will be modified as the
> -                     * translation buffer has been flushed.
> -                     */
> -                    last_tb = NULL;
> -                    cpu->tb_flushed = false;
> -                }
> -                /* see if we can patch the calling TB. When the TB
> -                   spans two pages, we cannot safely do a direct
> -                   jump. */
> -                if (last_tb != NULL && tb->page_addr[1] == -1
> -                    && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
> -                    tb_add_jump(last_tb, tb_exit, tb);
> -                }
> -                tb_unlock();
> +                tb = tb_find_fast(cpu, &last_tb, tb_exit);
>                  if (likely(!cpu->exit_request)) {
>                      uintptr_t ret;
>                      trace_exec_tb(tb, tb->pc);
>
> ... right?

Yeah that sort of thing.

>
> Kind regards,
> Sergey


--
Alex Bennée



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