Re: [Qemu-devel] [PATCH v4 23/26] tcg: Emit prologue to the beginning of code_gen_buffer

[Richard Henderson <rth@twiddle.net>]

On 10/01/2015 02:17 AM, Aurelien Jarno wrote:
> On 2015-09-30 15:09, Richard Henderson wrote:
>> By putting the prologue at the end, we risk overwriting the
>> prologue should our estimate of maximum TB size.  Given the
>> two different placements of the call to tcg_prologue_init,
>> move the high water mark computation into tcg_prologue_init.
>>
>> Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
>> Signed-off-by: Richard Henderson <rth@twiddle.net>
>> ---
>>   tcg/tcg.c       | 35 ++++++++++++++++++++++++++++-------
>>   translate-all.c | 28 +++++++++-------------------
>>   2 files changed, 37 insertions(+), 26 deletions(-)
>
> Good idea to move it. I have done some experiments with putting slow
> path "helpers" in the prologue, and I ended-up going over the 1024
> bytes limits.
>
>> diff --git a/tcg/tcg.c b/tcg/tcg.c
>> index d3693b1..5609108 100644
>> --- a/tcg/tcg.c
>> +++ b/tcg/tcg.c
>> @@ -363,17 +363,38 @@ void tcg_context_init(TCGContext *s)
>>
>>   void tcg_prologue_init(TCGContext *s)
>>   {
>> -    /* init global prologue and epilogue */
>> -    s->code_buf = s->code_gen_prologue;
>> -    s->code_ptr = s->code_buf;
>> +    size_t prologue_size, total_size;
>> +    void *buf0, *buf1;
>> +
>> +    /* Put the prologue at the beginning of code_gen_buffer.  */
>> +    buf0 = s->code_gen_buffer;
>> +    s->code_ptr = buf0;
>> +    s->code_buf = buf0;
>> +    s->code_gen_prologue = buf0;
>> +
>> +    /* Generate the prologue.  */
>>       tcg_target_qemu_prologue(s);
>> -    flush_icache_range((uintptr_t)s->code_buf, (uintptr_t)s->code_ptr);
>> +    buf1 = s->code_ptr;
>> +    flush_icache_range((uintptr_t)buf0, (uintptr_t)buf1);
>> +
>> +    /* Deduct the prologue from the buffer.  */
>> +    prologue_size = tcg_current_code_size(s);
>> +    s->code_gen_ptr = buf1;
>> +    s->code_gen_buffer = buf1;
>> +    s->code_buf = buf1;
>> +    total_size = s->code_gen_buffer_size - prologue_size;
>> +    s->code_gen_buffer_size = total_size;
>> +
>> +    /* Compute a high-water mark, at which we voluntarily flush the
>> +       buffer and start over.  */
>> +    s->code_gen_buffer_max_size = total_size - TCG_MAX_OP_SIZE * OPC_BUF_SIZE;
>> +
>> +    tcg_register_jit(s->code_gen_buffer, total_size);
>
> I am not sure why you moved this 2 lines there, I think they have more
> their place in code_gen_alloc() so that the heuristics stay at the same
> place. total_size is available in s->code_gen_buffer_size, so that
> should be doable.

Because it was done too early for user-only.  There, the sequence is

  code_gen_alloc
  place guest image, setting GUEST_BASE
  tcg_prologue_init

We need the guest base for generating the prologue on most targets.  Although 
clearly a better ordering is to place the guest image *first* before doing 
anything else.  None of the other large allocations (including tbs) really care 
where they're placed.

Something to fix later, or should I do a v5?


r~