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Re: vIOMMU - PCI pass through to Layer 2 VMs (Nested Virtualization)
From: |
Eric Auger |
Subject: |
Re: vIOMMU - PCI pass through to Layer 2 VMs (Nested Virtualization) |
Date: |
Mon, 9 Oct 2023 11:29:36 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 |
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
>
>