Re: [PATCH 0/4] cover letter: qemu: Implement support for iommufd

[Ján Tomko via Devel]

On a Monday in 2025, Nathan Chen via Devel wrote:

Hi,

This series implements support for using iommufd to propagate DMA
mappings to the kernel for VM-assigned host devices in a qemu VM.

We add a new 'iommufd' attribute for hostdev devices to be
associated with the iommufd object.

For instance, specifying the iommufd object and associated hostdev in a
VM definition:

 <devices>
...
   <hostdev mode='subsystem' type='pci' managed='no'>
     <driver iommufd='yes'/>
     <source>
       <address domain='0x0009' bus='0x01' slot='0x00' function='0x0'/>
     </source>
     <address type='pci' domain='0x0000' bus='0x15' slot='0x00' function='0x0'/>
   </hostdev>
   <hostdev mode='subsystem' type='pci' managed='no'>
     <driver iommufd='yes'/>
     <source>
       <address domain='0x0019' bus='0x01' slot='0x00' function='0x0'/>
     </source>
     <address type='pci' domain='0x0000' bus='0x16' slot='0x00' function='0x0'/>
   </hostdev>
...
 </devices>

Are there any hardware/kernel requirements, or something done to be
host? Even when I add the device to the vfio-pci driver, it does not
create /dev/vfio/devices for me:

error: unsupported configuration: VFIO device /dev/vfio/devices/vfio0 not found 
- ensure device is bound to vfio-pci driver

Kernel: 6.17.6-300.fc43.x86_64 on Fedora
QEMU: v10.1.0-2147-g917ac07f9a (the current master)

Also, I'd expect it to just work with managed='yes'.

Jano

This would get translated to a qemu command line with the arguments below.
Note that libvirt will open the /dev/iommu and VFIO cdev, passing the
associated fd number to qemu:

-object '{"qom-type":"iommufd","id":"iommufd0","fd":"24"}' \
-device 
'{"driver":"vfio-pci","host":"0009:01:00.0","id":"hostdev0","iommufd":"iommufd0","fd":"22","bus":"pci.21","addr":"0x0"}'
 \
-device 
'{"driver":"vfio-pci","host":"0019:01:00.0","id":"hostdev1","iommufd":"iommufd0","fd":"25","bus":"pci.22","addr":"0x0"}'
 \

This series is on Github:
https://github.com/NathanChenNVIDIA/libvirt/tree/iommufd-10-23-25

Thanks,
Nathan

Signed-off-by: Nathan Chen <nath...@nvidia.com>

Nathan Chen (4):
 qemu: Implement support for associating iommufd to hostdev
 qemu: open iommufd FDs from libvirt backend
 qemu: Update Cgroup, namespace, and seclabel for qemu to access
   iommufd paths
 tests: qemuxmlconfdata: provide iommufd sample XML and CLI args

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