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Date: Wed, 3 Oct 2018 14:07:31 -0400
From: "Emilio G. Cota" <cota@braap.org>
Message-ID: <20181003180731.GA26376@flamenco>
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Subject: Re: [Qemu-devel] [PATCH 2/3] cputlb: serialize tlb updates with
 env->tlb_lock
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Alex =?iso-8859-1?Q?Benn=E9e?= <alex.bennee@linaro.org>, qemu-devel@nongnu.org, Richard Henderson <rth@twiddle.net>

On Wed, Oct 03, 2018 at 19:05:51 +0200, Paolo Bonzini wrote:
> On 03/10/2018 19:02, Emilio G. Cota wrote:
> >> For reads I agree, but you may actually get a torn read if the writer
> >> doesn't use atomic_set.
> >
> > But you cannot get a torn read if all reads that don't hold the lock
> > are coming from the same thread that performed the write.
> 
> Ah, so you are relying on copy_tlb_helper(_locked) being invoked only
> from the vCPU thread (as opposed to someone else doing tlb_flush)?

Yes. tlb_flush_nocheck is always run by the owner thread--tlb_flush
checks for this, and if !qemu_cpu_is_self(cpu) then it schedules
async work on the owner thread.

> Maybe it's worth adding a comment if that's what I missed.

I'll send a v2 with an updated comment and a debug-only assert
in the copy helper.

Thanks,

		E.