[SRU][U/N][M][L][PATCH v2 0/1] Kernel config option missing for s390x PCI passthrough (LP: 2042853)

[frank.heimes at canonical.com]

BugLink: https://bugs.launchpad.net/bugs/2042853

SRU Justification:

[Impact]

 * Today no s390x-specific vfio-pci devices (zPCI) can be passed
   from a KVM host to a KVM guest (incl. secure execution guests
   in the context of confidential computing).

 * s390x PCI passthrough needs various changes in the s390x kernel zPCI
   code (incl. the new s390x-specific Kernel config option
   'CONFIG_VFIO_PCI_ZDEV_KVM') that were introduced with kernel 6.0
   and got backported to 22.04/jammy as part of LP: #1853306.

 * Lunar an newer Ubuntu releases have the code already included from
   upstream (incl. the Kernel option 'CONFIG_VFIO_PCI_ZDEV_KVM'), but the
   config option is not set, hence zPCI pass-through is still not possible.

[Fix]

 * To be able to make use of VFIO zPCI pass-through on s390x running newer
   Ubuntu releases (especially needed in the context of secure execution)
   the (s390x-specific) Kernel config option 'CONFIG_VFIO_PCI_ZDEV_KVM' needs
   to be enabled and set to 'y'.

[Test Case]

 * Hardware used: z14 or greater LPAR, PCI-attached devices
   (RoCE VFs, ISM devices, NVMe drive)

 * Setup: Both the kernel and QEMU features are needed for the feature
   to function (an upstream QEMU can be used to verify the kernel early),
   and the facility is only available on z14 or newer.
   When any of those pieces is missing,
   the interpretation facility will not be used.
   When both the kernel and QEMU features are included in their respective
   packages, and running in an LPAR on a z14 or newer machine,
   this feature will be enabled automatically.
   Existing supported devices should behave as before with no changes
   required by an end-user (e.g. no changes to libvirt domain definitions)
   -- but will now make use of the interpretation facility.
   Additionally, ISM devices will now be eligible for vfio-pci passthrough
   (where before QEMU would exit on error if attempting to provide an ISM
   device for vfio-pci passthrough, preventing the guest from starting)

 * Testing will include the following scenarios, repeated each for RoCE,
   ISM and NVMe:

   1) Testing of basic device passthrough (create a VM with a vfio-pci
      device as part of the libvirt domain definition, passing through
      a RoCE VF, an ISM device, or an NVMe drive. Verify that the device
      is available in the guest and functioning)
   2) Testing of device hotplug/unplug (create a VM with a vfio-pci device,
      virsh detach-device to remove the device from the running guest,
      verify the device is removed from the guest, then virsh attach-device
      to hotplug the device to the guest again, verify the device functions
      in the guest)
   3) Host power off testing: Power off the device from the host, verify
      that the device is unplugged from the guest as part of the poweroff
   4) Guest power off testing: Power off the device from within the guest,
      verify that the device is unusable in the guest,
      power the device back on within the guest and verify that the device
      is once again usable.
   5) Guest reboot testing: (create a VM with a vfio-pci device,
      verify the device is in working condition, reboot the guest,
      verify that the device is still usable after reboot)

[Regression Potential]

 * The regression potential is moderate, since the code is upstream
   for quite a while and already enabled in jammy.

 * The general way on using passthrough has not changed, with this
   change (config option) it's now just possible to passthrough
   zPCI on top.

 * CCW devices are not affected.

 * And this is s390x-specific anyway, so no other architectures are affected.

[Other]

 * The enablement of the kernel config option is the same for L, M and U/N,
   but I submitted separate patches due to slightly different contexts.

 * v2: - renamed patch to kernel option name
       - toggled existing option from n to y
       - reworked patch description

Frank Heimes (1):
  UBUNTU: [Config] CONFIG_VFIO_PCI_ZDEV_KVM=y

 debian.master/config/annotations | 3 +++
 1 file changed, 3 insertions(+)

-- 
2.25.1