Re: [Qemu-devel] [PATCH 21/22] tcg: enable per-thread TCG for softmmu

[Richard Henderson <rth@twiddle.net>]

On 07/09/2017 11:29 AM, Emilio G. Cota wrote:
> On Sun, Jul 09, 2017 at 11:19:37 -1000, Richard Henderson wrote:
>> On 07/08/2017 09:50 PM, Emilio G. Cota wrote:
>>> This allows us to generate TCG code in parallel. MTTCG already uses
>>> it, although the next commit pushes down a lock to actually
>>> perform parallel generation.
>>>
>>> User-mode is kept out of this: contention due to concurrent translation
>>> is more commonly found in full-system mode.
>>
>> Um, why do you believe that?  Are you suggesting that a multi-threaded
>> user-only guest is much more likely to share TBs and do much less code
>> generation total?
> 
> Exactly. Also, in user-mode "vCPU threads" (i.e. host threads) come and
> go all the time, so this doesn't work well with having a single
> code_gen_buffer, which I assumed was non-negotiable.

Ah, yes.  For any subdivision N of code_gen_buffer that we choose, at some 
point we may have N+1 threads and need to do Something Else.

That's probably something worth commenting somewhere with the "first" #ifndef 
CONFIG_USER_ONLY.


r~