From mboxrd@z Thu Jan  1 00:00:00 1970
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: [RFC 7/11] virtio_pci: new, capability-aware driver.
Date: Thu, 12 Jan 2012 07:46:04 +1100
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Pawel Moll <pawel.moll@arm.com>, "Michael S. Tsirkin" <mst@redhat.com>, virtualization <virtualization@lists.linux-foundation.org>, Christian Borntraeger <borntraeger@de.ibm.com>, Sasha Levin <levinsasha928@gmail.com>
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On Wed, 2012-01-11 at 14:28 +0000, Stefan Hajnoczi wrote:
> On Wed, Jan 11, 2012 at 9:10 AM, Benjamin Herrenschmidt
> <benh@kernel.crashing.org> wrote:
> > On Wed, 2012-01-11 at 08:47 +0000, Stefan Hajnoczi wrote:
> >>
> >> This is also an opportunity to stop using CPU physical addresses in
> >> the ring and instead perform DMA like a normal PCI device (use bus
> >> addresses).
> >
> > Euh why ?
> 
> Because it's a paravirt hack that ends up hitting corner cases.  It's
> not possible to do virtio-pci passthrough under nested virtualization
> unless we use an IOMMU.  Imagine passing virtio-net from L0 into the
> L2 guest (i.e. PCI-passthrough).  If virtio-pci is really "PCI" this
> should be possible but it's not when we use physical addresses instead
> of bus addresses.

Is this just an academic exercise or is there any actual value in doing
this ?

Using an iommu is going to slaugher your performance, so at the very
least it should be kept an option.

Yes, it's a paravirt "hack" as you call it but that's what virtio is all
about.... paravirt. If you prefer you can emulate a real HW device :-)

Ben.