question about virtio related options: ats=on and iommu=on

[Yalan Zhang]

Hi,

I have a question about the virtio related options, could someone please
help to confirm?
In my understanding, the ats='on' should depend on the "iommu=on", but I'm
not sure about it.
Please check the details below.
Thank you!

Details:
1. set vm with intel iommu device, and enable iommu for virtio  filesystem
device:
# virsh dumpxml rhel  --xpath //iommu
<iommu model="intel">
  <driver intremap="on" caching_mode="on" iotlb="on"/>
</iommu>

Set iommu="on" ats="on" for virtio filesystem device:
# virsh dumpxml rhel --xpath //filesystem
<filesystem type="mount" accessmode="passthrough">
  <driver type="virtiofs" queue="512" iommu="on" ats="on"/>
  <binary path="/usr/libexec/virtiofsd" xattr="on">
    <cache mode="none"/>
  </binary>
  <source dir="/path1"/>
  <target dir="mount_tag1"/>
  <address type="pci" domain="0x0000" bus="0x04" slot="0x00"
function="0x0"/>
</filesystem>

2. failed to start the vm since "iommu_platform=true is not supported by
the device"
# virsh start rhel
error: Failed to start domain 'rhel'
error: internal error: qemu unexpectedly closed the monitor:
2023-01-09T03:48:03.198629Z qemu-kvm: -device
{"driver":"vhost-user-fs-pci","iommu_platform":true,"ats":true,"id":"fs0","chardev":"chr-vu-fs0","queue-size":512,"tag":"mount_tag1","bus":"pci.4","addr":"0x0"}:
iommu_platform=true is not supported by the device

3. update the xml to be with only ats="on", vm can start successfully,
which is not expected.
# virsh dumpxml rhel --xpath //filesystem
<filesystem type="mount" accessmode="passthrough">
  <driver type="virtiofs" queue="512" ats="on"/>
  <binary path="/usr/libexec/virtiofsd" xattr="on">
    <cache mode="none"/>
  </binary>
  <source dir="/path1"/>
  <target dir="mount_tag1"/>
  <alias name="fs0"/>
  <address type="pci" domain="0x0000" bus="0x04" slot="0x00"
function="0x0"/>
</filesystem>

the qemu cmd line:
-chardev
socket,id=chr-vu-fs0,path=/var/lib/libvirt/qemu/domain-5-rhel/fs0-fs.sock \
-device
'{"driver":"vhost-user-fs-pci","ats":true,"id":"fs0","chardev":"chr-vu-fs0","queue-size":512,"tag":"mount_tag1","bus":"pci.4","addr":"0x0"}'

Refer to libvirt.org:
QEMU's virtio devices have some attributes related to the virtio transport
under the driver element: The iommu attribute enables the use of emulated
IOMMU by the device.
The attribute ats controls the Address Translation Service support for PCIe
devices. This is needed to make use of IOTLB support (see IOMMU devices).
Possible values are on or off.

In my understanding, the ats='on' should depend on the iommu='on', so if
the device does not support iommu, it should not support ats, eigher.
I'm not sure if the understanding is correct.
Could someone please help to confirm it?
Thank you!


Yalan
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