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From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: Sun, 11 Mar 2012 13:02:47 +1100
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Subject: Re: [Qemu-devel] [PATCH 13/13] pseries: Implement IOMMU and DMA for
 PAPR PCI devices
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To: David Gibson <david@gibson.dropbear.id.au>
Cc: mst@redhat.com, qemu-devel@nongnu.org, agraf@suse.de, eduard.munteanu@linux360.ro, Paolo Bonzini <pbonzini@redhat.com>, rth@twiddle.net

On Fri, 2012-03-09 at 21:58 +1100, David Gibson wrote:
> > What about virtio?
> 
> virtio doesn't use virtualized PCI DMA, it uses direct hypervisor
> access to guest memory, by guest physical address.  It *shouldn't*,
> but it does - that's the way it's specced and that's the way the guest
> kernel expects it to be.  It could be fixed with a new feature bit,
> but that's a project for another day.

More precisely, the patch don't break virtio as virtio just bypasses
this all.

Also having virtio go through the iommu might not be such a great idea,
it should definitely remain optional. The ability of virtio to go
straight to guest memory has some significant performance advantages.

Cheers,
Ben.