Re: [PATCH v4 0/3] qemu: Support vCMDQ

19 Aug 2026


      On Tue, Aug 18, 2026 at 11:30:09AM -0700, Nathan Chen via Devel wrote:
...
Hi,
This is a follow up to the previous patchset [0] for supporting Tegra241
vCMDQ.
This patchset is tested with Shameer's qemu series for vCMDQ support.
The accelerated SMMUv3 series for qemu and Libvirt are
pre-requisites for vCMDQ support, as accelerated SMMUv3 must be enabled
alongside vCMDQ.
A new 'cmdqv' <iommu> driver attribute is introduced to toggle enabling
vCMDQ support. For instance, specifying hostdevs associated with
multiple accelerated SMMUs + CMDQV enabled, configured to be routed to
pcie-expander-bus controllers in a way where VFIO device to SMMUv3
associations are matched with the host:
<devices>
...
    <controller type='pci' index='1' model='pcie-expander-bus'>
      <model name='pxb-pcie'/>
      <target busNr='252'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x0'/>
    </controller>
    <controller type='pci' index='2' model='pcie-expander-bus'>
      <model name='pxb-pcie'/>
      <target busNr='248'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </controller>
...
    <controller type='pci' index='21' model='pcie-root-port'>
      <model name='pcie-root-port'/>
      <target chassis='21' port='0x0'/>
      <address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
    </controller>
    <controller type='pci' index='22' model='pcie-root-port'>
      <model name='pcie-root-port'/>
      <target chassis='22' port='0xa8'/>
      <address type='pci' domain='0x0000' bus='0x02' slot='0x00' function='0x0'/>
    </controller>
...
    <hostdev mode='subsystem' type='pci' managed='no'>
      <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'>
      <source>
        <address domain='0x0019' bus='0x01' slot='0x00' function='0x0'/>
      </source>
      <address type='pci' domain='0x0000' bus='0x16' slot='0x00' function='0x0'/>
    </hostdev>
    <iommu model='smmuv3'>
      <driver pciBus='1' accel='on' ats='on' ril='on' ssidsize='20' oas='44' cmdqv='on'/>
    </iommu>
    <iommu model='smmuv3'>
      <driver pciBus='2' accel='on' ats='on' ril='on' ssidsize='20' oas='44' cmdqv='on'/>
    </iommu>
  </devices>
This would get translated to a qemu command line with the arguments
below:
-device '{"driver":"pxb-pcie","bus_nr":252,"id":"pci.1","bus":"pcie.0","addr":"0x1"}' \
 -device '{"driver":"pxb-pcie","bus_nr":248,"id":"pci.2","bus":"pcie.0","addr":"0x2"}' \
 -device '{"driver":"pcie-root-port","port":0,"chassis":21,"id":"pci.21","bus":"pci.1","addr":"0x0"}' \
 -device '{"driver":"pcie-root-port","port":168,"chassis":22,"id":"pci.22","bus":"pci.2","addr":"0x0"}' \
 -device '{"driver":"arm-smmuv3","primary-bus":"pci.1","id":"iommu0","accel":true,"ats":true,"cmdqv":"on","ril":true,"pasid":true,"oas":44}' \
 -device '{"driver":"arm-smmuv3","primary-bus":"pci.2","id":"iommu1","accel":true,"ats":true,"cmdqv":"on","ril":true,"pasid":true,"oas":44}' \
 -device '{"driver":"vfio-pci","host":"0009:01:00.0","id":"hostdev0","bus":"pci.21","addr":"0x0"}' \
 -device '{"driver":"vfio-pci","host":"0019:01:00.0","id":"hostdev1","bus":"pci.22","addr":"0x0"}' \
This series is on Github:
https://github.com/NathanChenNVIDIA/libvirt/tree/cmdqv-v4/
Thanks,
Nathan
Changes from v3:
- Move qemuValidateDomainDeviceDefIOMMU() check from patch 1 to 2
- Mention default hypervisor value will be used in formatdomain.rst if
  cmdqv is omitted
- Make cmdqv virDomainIOMMUDefValidate() check aligned with order of all
  cmdqv checks in the function
- Include cmdqv for accel validation error message in domain_validate.c
Changes from v2:
- Add QEMU capabilities support for cmdqv
- Change QEMU command line parameter to expect OnOffAuto string
Changes from v1:
- Remove 'identifier' attribute for SMMUv3
- Change QEMU command line property from 'tegra241-cmdqv' to 'cmdqv'
- Support QEMU OnOffAuto CMDQV property instead of expecting a bool
[0] https://lists.libvirt.org/archives/list/devel@lists.libvirt.org/thread/NGSB6...
Nathan Chen (3):
  qemu: introduce QEMU_CAPS_ARM_SMMUV3_CMDQV
  qemu: add IOMMU attribute "cmdqv" for smmuv3
  tests: qemuxmlconfdata: provide cmdqv sample XML and CLI args
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>