Re: vIOMMU - PCI pass through to Layer 2 VMs (Nested Virtualization)

[Eric Auger <eauger@redhat.com>]

Hi Markus,

On 10/9/23 09:06, Markus Frank wrote:
> Hello,
> 
> I have already sent this email to qemu-discuss but I did not get a reply.
> https://lists.nongnu.org/archive/html/qemu-discuss/2023-09/msg00034.html
> Maybe someone here could help me and reply to this email or the one on
> qemu-discuss?
> 
> I would like to pass through PCI devices to Layer-2 VMs via Nested
> Virtualization.
> 
> Is there current documentation for this topic somewhere?
> 
> I used these parameters:
> -machine ...,kernel-irqchip=split
> -device intel-iommu
> 
> With these parameters PCI pass through to L2-VMs worked fine.
> 
> 
> Now I come to the part where I get confused.
> 
> https://wiki.qemu.org/Features/VT-d#With_Virtio_Devices
> Is this documentation relevant for PCI pass through? Do I need DMAR for
> virtio devices?
If you just want the host assigned devices to be protected by the
viommu, you don't need to add iommu_platform=on along with the
virtio-pci devices.
> 
> And there is also the virtio-iommu device where I also could use the
> i440fx chipset.
> https://michael2012z.medium.com/virtio-iommu-789369049443

you can use virtio-iommu with q35 machine.
> 
> When adding "-device virtio-iommu-pci" pci pass through also works
> but I get "kvm: virtio_iommu_translate no mapping for 0x1002030f000 for
> sid=240"
> when starting qemu. What could that mean?
Normally you shouldn't get any such error. This means there is no
mapping programmed by the iommu-driver for this requester id (0x240) and
this iova=0x1002030f000. But if I understand correctly this does not
prevent your device from working, correct?
> 
> What do these parameters
> "disable-legacy=on,disable-modern=off,iommu_platform=on,ats=on"
> actually do? When do I need them and on which virtio devices?
you need them if you want your virtio devices to be protected by the
viommu. Otherwise the viommu is bypassed.
> 
> And which device should I rather use: virtio-iommu or intel-iommu?
Both should be working. virtio-iommu is more recent and less used in
production than intel-iommu though.

Thanks

Eric
> 
> Thanks in advance,
> Markus
> 
>