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From: Peter Delevoryas <peter@pjd.dev>
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Subject: [q&a] Status of IOMMU virtualization for nested virtualization
 (userspace PCI drivers in VMs)
Message-Id: <3D96D76D-85D2-47B5-B4C1-D6F95061D7D6@pjd.dev>
Date: Wed, 28 Feb 2024 10:29:32 -0800
Cc: qemu-devel <qemu-devel@nongnu.org>,
 suravee.suthikulpanit@amd.com,
 iommu@lists.linux.dev,
 alex.williamson@redhat.com,
 kvm@vger.kernel.org,
 linux-kernel@vger.kernel.org

Hey guys,

I=E2=80=99m having a little trouble reading between the lines on various =
docs, mailing list threads, KVM presentations, github forks, etc, so I =
figured I=E2=80=99d just ask:

What is the status of IOMMU virtualization, like in the case where I =
want a VM guest to have a virtual IOMMU?

I found this great presentation from KVM Forum 2021: [1]

1. I=E2=80=99m using -device intel-iommu right now. This has performance =
implications and large DMA transfers hit the vfio_iommu_type1 =
dma_entry_limit on the host because of how the mappings are made.

2. -device virtio-iommu is an improvement, but it doesn=E2=80=99t seem =
compatible with -device vfio-pci? I was only able to test this with =
cloud-hypervisor, and it has a better vfio mapping pattern (avoids =
hitting dma_entry_limit).

3. -object iommufd [2] I haven=E2=80=99t tried this quite yet, planning =
to: if it=E2=80=99s using iommufd, and I have all the right kernel =
features in the guest and host, I assume it=E2=80=99s implementing the =
passthrough mode that AMD has described in their talk? Because I imagine =
that would be the best solution for me, I=E2=80=99m just having trouble =
understanding if it=E2=80=99s actually related or orthogonal. I see AMD =
has -device amd-viommu here [3], is that ever going to be upstreamed or =
is that what -object iommufd is abstracting? I also found this mailing =
list submission [4], and the context and changes there imply this is all =
about that (exposing iommu virtualization to the guest)

Thanks!
Peter
=20
[1] =
https://static.sched.com/hosted_files/kvmforum2021/da/vIOMMU%20KVM%20Forum=
%202021%20-%20v4.pdf
[2] https://www.qemu.org/docs/master/devel/vfio-iommufd.html
[3] =
https://github.com/AMDESE/qemu/commit/ee056455c411ee3369a47c65ba8a54783b5d=
2814
[4] =
https://lore.kernel.org/lkml/20230621235508.113949-1-suravee.suthikulpanit=
@amd.com/