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Re: [Xen-devel] TLB flushing in blktap kernel driver?

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] TLB flushing in blktap kernel driver?
From: "Stephen C. Tweedie" <sct@xxxxxxxxxx>
Date: Wed, 27 Sep 2006 14:33:44 +0100
Cc: Andrew Warfield <andrew.warfield@xxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Julian Chesterfield <jac90@xxxxxxxxx>
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On Wed, 2006-09-27 at 14:30 +0100, Keir Fraser wrote:
> > However, I can't understand right now why we need the tlb flush here.
> > If we're doing any grant table operations between dom0 and domU, then
> > the HV should be doing the tlb flushes for us, shouldn't it?  And mere
> > SMP synchronisation on a ring buffer requires a physical memory barrier,
> > not a TLB synchronisation (and RING_PUSH_REQUESTS already does a wmb.)
> Grant unmapping certainly guarantees to flush TLB entries.

Right.  I can't see why a simple ring buffer poll would *ever* need to
flush tlbs.  I may be missing something, which is why I asked, but it
looks like this was perhaps just intended to be a memory barrier, in
which case the flush call is entirely superfluous (and a significant
performance hit to boot.)


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