Re: [RFC 7/11] virtio_pci: new, capability-aware driver.

[Stefan Hajnoczi <stefanha@xxxxxxxxx>]

On Wed, Jan 11, 2012 at 9:10 AM, Benjamin Herrenschmidt
<benh@xxxxxxxxxxxxxxxxxxx> 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
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