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

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On Wed, Jan 11, 2012 at 8:46 PM, Benjamin Herrenschmidt
<benh@xxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, 2012-01-11 at 14:28 +0000, Stefan Hajnoczi wrote:
>> 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.
> Is this just an academic exercise or is there any actual value in doing
> this ?

It's a corner case, the value is small.  I also hit this with virtio
on SPARC which is made difficult by the fact that the Solaris kernel
assumes there is an IOMMU for scatter-gather and doesn't provide
functions for allocating physically contiguous memory in drivers.
It's another instance where this shortcut comes up against problems
and behaving like a real PCI device would work fine.

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

That's a good idea.  By default it can continue to use physical addresses.

I guess there's no point in worrying about it until we have a real user.

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