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

[Paolo Bonzini]

> I agree that it would be nice to have the same mechanism for all.
> 
> The main hurdle I see is how to allow for concurrent code generation while
> minimizing flushes of the single, fixed-size[*] code_gen_buffer.
> In user-mode this is tricky because there is no way to bound the number
> of threads that might be spawned by the guest code (I don't think reading
> /proc/sys/kernel/threads-max is a viable solution here).
> 
> Switching to a "__thread *tcg_ctx_ptr" model will help minimize
> user-mode/softmmu differences though. The only remaining difference would be
> that user-mode would need tb_lock() around tb_gen_code, whereas softmmu
> wouldn't, but everything else would be the same.

Hmm, tb_gen_code is already protected by mmap_lock in linux-user, so you 
wouldn't
get any parallelism.  On the other hand, you could just say that the fixed-size
code_gen_buffer is protected by mmap_lock, which doesn't exist for softmmu.

Paolo