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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 16:44:25 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

On 31/03/2016 16:35, Sergey Fedorov wrote:
>> > 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.
> Maybe save this idea for latter? :) We'd better use a simpler approach
> at first and then move on and optimize. BTW, a few years ago I came
> across an interesting paper on code cache eviction granularities [1].

It depends on what is simpler.  Emilio and Fred's code was tricky and
touched the central CPU execution loop in cpus.c.


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