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

[Richard Henderson]

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?

At the moment I think it's just a confusing distinction.  As proven by some of 
the comment adjustments you made.

> -TCGContext tcg_ctx;
> +TCG_THREAD TCGContext tcg_ctx;

This is a really large structure, and it's not needed by any of the I/O 
threads.  We're probably better off dynamically allocating this ourselves and 
do something like

__thread TCGContext *tcg_ctx_ptr;
#define tcg_ctx (*tcg_ctx_ptr)

You could then even have tcg_init_ctx be the object instead of a pointer to the 
object.

for the main thread, so you don't have to have an extra pointer to this; just 
reference it by symbol.

> -static struct tcg_temp_info temps[TCG_MAX_TEMPS];
> -static TCGTempSet temps_used;
> +static TCG_THREAD struct tcg_temp_info temps[TCG_MAX_TEMPS];
> +static TCG_THREAD TCGTempSet temps_used;


I've been meaning to dynamically allocate these for a while now.  I think that 
would be better than putting this large array in TLS.  Which, again, is not 
needed for I/O threads.


r~