From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stefan Hajnoczi <stefanha@gmail.com>
Subject: Re: [RFC 7/11] virtio_pci: new, capability-aware driver.
Date: Wed, 11 Jan 2012 14:28:48 +0000
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To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
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, 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.

Stefan