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

[Frank Heimes]

Okay, I see and agree. Thx for the feedback.

Will provide a v2 soon ...


On Tue, Nov 21, 2023 at 9:38 AM Stefan Bader <stefan.bader at canonical.com>
wrote:

> On 17.11.23 16:28, frank.heimes at canonical.com wrote:
> > 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 enabling of the kernel config option is exactly the same for L, M
> >     and U/N, but I submitted separate patches due to slightly different
> context
> >     and offsets.
> >
> > Frank Heimes (1):
> >    UBUNTU: [Config] enable VFIO zPCI pass-through for s390x
> >
> >   debian.master/config/annotations | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> I probably would rather reject for the following reasons:
> * The config option already exists at least in the M/L annotations
> * Those should be modified instead of adding duplicate entries
> * I would expect that if this patch is applied and updateconfigs is run
> then
>    lines will get moved around.
> I would rather avoid this when starting to prep  a kernel. So please
> double check and in case this happens submit a new set.
>
> Thanks,
> -Stefan
> --
> - Stefan
>
>
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