Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Jul 28, 2015 at 9:44 AM, Jan Kiszka <jan.kiszka@siemens.com> wrote:
> The ability to have virtio on systems with IOMMU in place makes testing
> much more efficient for us. Ideally, we would have it in non-identity
> mapping scenarios as well, e.g. to start secondary Linux instances in
> the test VMs, giving them their own virtio devices. And we will
> eventually have this need on ARM as well.
>
> Virtio needs to be backward compatible, so the change to put these
> devices under IOMMU control could be advertised during feature
> negotiations and controlled on QEMU side via a device property. Newer
> guest drivers would have to acknowledge that they support virtio via
> IOMMUs. Older ones would refuse to work, and the admin could instead
> spawn VMs with this feature disabled.
>

The trouble is that this is really a property of the bus and not of
the device.  If you build a virtio device that physically plugs into a
PCIe slot, the device has no concept of an IOMMU in the first place.
Similarly, if you take an L0-provided IOMMU-supporting device and pass
it through to L2 using current QEMU on L1 (with Q35 emulation and
iommu enabled), then, from L2's perspective, the device is 1:1 no
matter what the device thinks.

IOW, I think the original design was wrong and now we have to deal
with it.  I think the best solution would be to teach QEMU to fix its
ACPI tables so that 1:1 virtio devices are actually exposed as 1:1.

--Andy