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From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 21/22] tcg: enable per-thread TCG for
 softmmu
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To: "Emilio G. Cota" <cota@braap.org>
Cc: qemu-devel@nongnu.org, Richard Henderson <rth@twiddle.net>


> 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