From mboxrd@z Thu Jan  1 00:00:00 1970
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: [RFC] virtio: use mandatory barriers for remote processor vdevs
Date: Sat, 03 Dec 2011 10:09:44 +1100
Message-ID: <1322867384.11728.20.camel@pasglop>
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To: Ohad Ben-Cohen <ohad@wizery.com>
Cc: kvm@vger.kernel.org, "Michael S. Tsirkin" <mst@redhat.com>, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, linux-arm-kernel@lists.infradead.org
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On Tue, 2011-11-29 at 14:31 +0200, Ohad Ben-Cohen wrote:
> Virtio is using memory barriers to control the ordering of
> references to the vrings on SMP systems. When the guest is compiled
> with SMP support, virtio is only using SMP barriers in order to
> avoid incurring the overhead involved with mandatory barriers.
> 
> Lately, though, virtio is being increasingly used with inter-processor
> communication scenarios too, which involve running two (separate)
> instances of operating systems on two (separate) processors, each of
> which might either be UP or SMP.
> 
> To control the ordering of memory references when the vrings are shared
> between two external processors, we must always use mandatory barriers.
> 
> A trivial, albeit sub-optimal, solution would be to simply revert
> commit d57ed95 "virtio: use smp_XX barriers on SMP". Obviously, though,
> that's going to have a negative impact on performance of SMP-based
> virtualization use cases.

Have you measured the impact of using normal barriers (non-SMP ones)
like we use on normal HW drivers unconditionally ?

IE. If the difference is small enough I'd say just go for it and avoid
the bloat.

Ben.