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Date: Tue, 25 Aug 2015 18:49:54 -0400 (EDT)
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [RFC 26/38] cpu: protect tb_jmp_cache with seqlock
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To: "Emilio G. Cota" <cota@braap.org>
Cc: mttcg@greensocs.com, mark burton <mark.burton@greensocs.com>, a rigo <a.rigo@virtualopensystems.com>, qemu-devel@nongnu.org, guillaume delbergue <guillaume.delbergue@greensocs.com>, alex bennee <alex.bennee@linaro.org>, Frederic Konrad <fred.konrad@greensocs.com>

> The seqlock for tb_jmp_cache is necessary the moment that the
> array can be wiped out with a memset(), as shown above. That
> function (tb_flush_jmp_cache) is called by tlb_flush_page,
> which has many callers.
> 
> One could argue that we could enforce calling tlb_flush_page to be
> a) always done by the owner thread or b) done while all others CPUs
> are paused.
> 
> I argue that worrying about that is not worth it; let's protect
> the array with a seqlock, which on TSO is essentially free, and
> worry about more important things.

Got it, this makes sense.

Paolo

> Wrt the next two patches:
> 
> Patch 27 is an improvement in that each TB has its own valid flag,
> which makes sense because this should only affect TB's that are
> trying to chain to/from it, not all TBs.
> 
> Patch 28 uses the RCU QLIST which to me seems cleaner and less
> error-prone than open-coding an RCU LIST.
> 
> Thanks,
> 
> 		Emilio
>