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 17:21:53 +0000
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Pawel Moll <pawel.moll@arm.com>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, 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 3:39 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Wed, Jan 11, 2012 at 02:28:48PM +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. =A0It's
>> not possible to do virtio-pci passthrough under nested virtualization
>> unless we use an IOMMU. =A0Imagine passing virtio-net from L0 into the
>> L2 guest (i.e. PCI-passthrough). =A0If virtio-pci is really "PCI" this
>> should be possible but it's not when we use physical addresses instead
>> of bus addresses.
>>
>> Stefan
>
> It won't be hard to show siginificant performance regression if
> we do this. Hard to justify for something as niche as nested virt.

For x86 this should be mostly a nop.  For ppc and SPARC architectures
maybe you're right.  I still think it's a design flaw because if
virtio v2 doesn't use bus addresses then it will simply not be
possible to do passthrough for nested virt and other cases we haven't
hit yet.

Stefan