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Re: [Qemu-devel] tcg: reworking tb_invalidated_flag

From: Paolo Bonzini
Subject: Re: [Qemu-devel] tcg: reworking tb_invalidated_flag
Date: Thu, 31 Mar 2016 15:40:11 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

On 31/03/2016 15:14, Sergey Fedorov wrote:
> On 30/03/16 21:13, Paolo Bonzini wrote:
>> On 30/03/2016 19:08, Sergey Fedorov wrote:
>>> The second approach is to make 'tb_invalidated_flag' per-CPU. This
>>> would be conceptually similar to what we have, but would give us thread
>>> safety. With this approach, we need to be careful to correctly clear and
>>> set the flag.
>> You can just ensure that setting and clearing it is done under tb_lock.
> So it could remain sitting in 'tcg_ctx.tb_ctx'. I'm just wondering what
> could be real benefits for making it per-CPU then?

All CPUs need to observe it in order to clear their own local next_tb
variable.  It is not enough to do that once, so it has to be per-CPU.

>> Because TranslationBlocks live in tcg_ctx.tb_ctx.tbs you need
>> special code to exit all CPUs at tb_flush time, otherwise you risk that
>> a tb_alloc reuses a TranslationBlock while it is in use by a VCPU.
> Looks like no matter which approach we use, it's ultimately necessary to
> ensure all CPUs have exited from translated code before the translation
> buffer may be safely flushed.

My plan was to use some kind of double buffering, where only half of
code_gen_buffer is in use.  At the end of tb_flush you call cpu_exit()
on all CPUs, so that CPUs stop executing chained TBs from the old half
before they can see one from the new half.

If code_gen_buffer is static you have to preallocate two buffers (and
two tbs arrays) and waste one of them; while it is theoretically
possible to have CPUs still executing from the old half while you finish
the new half, it can be more or less ignored.

If it is dynamic, the previously used areas can be freed with call_rcu,
and you can safely allocate a new code_gen_buffer and tbs array.

I haven't thought much about it; it might require keeping a cache of the
tbs array per CPU, and possibly changing the code under "if
(tcg_ctx.tb_ctx.tb_invalidated_flag)" to simply exit cpu_exec.


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