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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2015-07-28 19:10, Andy Lutomirski wrote:
> 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.

If one would build a real virtio device today, it would be broken
because every IOMMU would start to translate its requests. Already from
that POV, we really need to introduce a feature flag "I will be
IOMMU-translated" so that a potential physical implementation can carry
it unconditionally.

> 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.

Only the current drivers are broken. And we can easily tell them apart
from newer ones via feature flags. Sorry, don't get the problem.

Jan

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