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RE: How to express "externally managed" IOMMU domains for VFIO/IOMMUFD ?
- To: Teddy Astie <teddy.astie@xxxxxxxxxx>, "iommu@xxxxxxxxxxxxxxx" <iommu@xxxxxxxxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>
- From: "Tian, Kevin" <kevin.tian@xxxxxxxxx>
- Date: Thu, 23 Apr 2026 08:01:50 +0000
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- Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 23 Apr 2026 08:02:07 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Thread-index: AQHc0nD/HbUzotrTgkKZyZa10hSZGrXsSRwA
- Thread-topic: How to express "externally managed" IOMMU domains for VFIO/IOMMUFD ?
> From: Teddy Astie
> Sent: Wednesday, April 22, 2026 11:59 PM
>
> Hello,
>
> On Xen, for PV-IOMMU [1], we have IOMMU support in Dom0, which in
> particular allows using VFIO and IOMMUFD from Dom0.
>
> However, its interactions with PCI Passthrough are unclear, and it would
VFIO manages PCI passthrough. since it's already allowed which part of
interaction is unclear?
> be preferable to let the kernel handle some of this logic. That would
> for instance avoid situations where toolstack causes Xen and Linux to go
> out of sync on where devices belong.
what is 'some of this logic' and what is the exact out-of-sync scenario?
>
> On Xen, we have a dedicated hypercalls for moving a device into another
> guest (so it no longer belongs in Dom0, at far as DMA is concerned).
>
> But it looks like there are no way to describe that idea of "attach that
> device to this VM" nor "the device is in a VM"; which makes that
> impracticable.
>
> There may be things that could be done with the vIOMMU objects, but
> there would be no "parent domain" in such case, as said earlier it
> doesn't exist in the IOMMU subsystem.
>
> What is expected to be done instead ?
>
> Teddy
>
> [1] https://www.youtube.com/watch?v=pLMGRgEJ-Eg
>
It'd be much easier to collect comments if you can put plain words
to explain the problem rather than expecting other folks to watch
the video first...
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