[SRU][N/O][PATCH 0/1] s390/pci: Fix zpci_bus_is_isolated_vf() for non-VFs

[Zixing Liu]

BugLink: https://bugs.launchpad.net/linux/+bug/2110289

SRU Justification:

[Impact]

 * For non-VFs, function zpci_bus_is_isolated_vf() should return false,
   because they aren't VFs.
   While zpci_iov_find_parent_pf() specifically checks if a function is a VF,
   it then simply returns that there is no parent.

 * The simplistic check for a parent then leads to these functions being
   confused with isolated VFs, and isolating them on their own domain even
   if sibling PFs should share the domain.

[Fix]

 * This is fixed by explicitly checking if a function is not a VF.
   (Notice that at this point the case where RIDs are ignored is already
    handled - and in this case all PCI functions get isolated by being
    properly detected in zpci_bus_is_multifunction_root().)

 * 8691abd3afaa "s390/pci: Fix zpci_bus_is_isolated_vf() for non-VFs"

[Test Plan]

 * Setup Ubuntu Server (24.04/24.10) for s390x on an IBM z17
   or LinuxONE 5 LPAR.

 * Have at least two PCIe adapters that support PF/VF (like RoCE Express)
   available in that LPAR.

 * Attach multiple PFs of a PF access mode device (notice that this is only
   possible with z17 and L1-5 hardware), such as the two PFs of a NETD
   to the same LPAR.

 * Observe that they are put into separate PCI domains
   instead of sharing the same domain as expected by drivers.

 * Due to lack of hardware, the verification will be conducted by IBM.

 * The fix was discussed upstream and flagged a stable kernel update.

[Where problems could occur]

 * The modification is limited to one additional if statement
   across two lines) in function zpci_bus_is_isolated_vf()
   in file arch/s390/pci/pci_bus.c.

 * Hence the modification will be limited to the s390x-specific parts of
   the PCI code in the kernel (sometimes refers to as zPCI),
   and will NOT impact any other architecture!

 * If add. if-statement is not correct and a wrong bool is returned,
   function zpci_bus_is_isolated_vf() might report and incorrect zpci_bus
   status. Either isolated when it's not or not-isolated when it really is.

 * And since the new if statement got inserted before the already existing
  'if (!pdev)', the latter code in function zpci_bus_is_isolated_vf()
   might be accidentally skipped.

 * All this might lead to a similar confusion of the functions in regard to
   isolated VFs status and whether isolating them on their own domain even
   or not, hence proper testing is needed.

[Other Info]

 * Since the commit got upstream accepted with v6.15-rc1,
   'Questing' is not affected.

 * Plucky got fixed with
   https://bugs.launchpad.net/bugs/2108854
   (cherry-picked as https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/plucky/commit/?id=58412b2aa12aa43c90d5414e22898a041ec05e8a)

 * Hence the only remaining affected releases, that contain the offending
   commit ("s390/pci: Fix handling of isolated VFs"), are oracular and noble.

 * Since this issue does not happen (thus cannot be recreated nor tested)
   on the IBM Z hardware we have in Canonical, the verification(s) will be
   done by IBM on most recent hardware.

Niklas Schnelle (1):
  s390/pci: Fix zpci_bus_is_isolated_vf() for non-VFs

 arch/s390/pci/pci_bus.c | 3 +++
 1 file changed, 3 insertions(+)

-- 
2.48.1