Re: [PATCH v7 0/4] vfio-pci/zdev: Improved zPCI Function Measurement Support

[Omar Elghoul <oelghoul@linux.ibm.com>]

Polite ping c:

Thanks

On 9/22/26 3:51 PM, Omar Elghoul wrote:
> Hi,
> 
> This patch series improves support for function measurement for zPCI
> passthrough devices on s390.
> 
> Changelog
> =========
> v6 -> v7:
> * Patch 2/4:
>    - Don't re-enable FMB if it wasn't already enabled
> 
> v5 -> v6:
> * Patch 2/4:
>    - Rework the FMB re-enablement code to reuse the same buffer again
>    - Make the FMB buffer persistent once allocated for as long as the
>      device's lifetime to accommodate an architectural quirk
> * Patch 4/4:
>    - Update the liveness check to use the new FMB enabled bool
> 
> v4 -> v5:
> * Typo in the cover letter
> * Swap the ordering of patches 3/4 and 4/4 to ease merging (i.e., to
>    ensure the three s390 patches are ordered before the VFIO patch)
> * Patch 2/4:
>    - Drop the refactor of zpci_fmb_enable_device() and the separation of
>      zpci_fmb_clear_iommu_ctrs() and zpci_fmb_do_enable()
>    - Allocate a new buffer in zpci_fmb_reenable_device() rather than
>      reusing the same buffer to avoid firmware edge cases
> 
> * Patch 3/4 (previously 4/4):
>    - Avoid reading from userspace while holding kzdev_lock unnecessarily
> 
> * Patch 4/4 (previously 3/4):
>    - Drop allowing usercopy of the FMB when initializing the kmem_cache
>    - Avoid copying to userspace while holding fmb_lock unnecessarily
>      - Restore the FMB bounce buffer to achieve this one
>    - Clarify uAPI documentation and ensure it accurately describes the
>      behavior of the VFIO features
> 
> v3 -> v4:
> * Patch 2/4:
>    - Replace mutex_lock/unlock in zpci_reenable_device() with a guard
> 
> * Patch 3/4:
>    - Allow usercopy of the FMB when initializing its kmem_cache
>    - Move the guard in vfio_pci_zdev_feature_fmb_enable() lower to only
>      protect the FMB
>    - Ensure vfio_pci_zdev_feature_fmb_enable() fails on double-enable for
>      consistency with the documentation
>    - Eliminate the bounce buffer in vfio_pci_zdev_feature_fmb_read()
>    - Replace the void pointer with __aligned_u64 in the FMB read uAPI
>      structure
> 
> v2 -> v3:
> * Patch 1/4 (new patch):
>    - Fix race conditions in pcibios_enable/disable_device() with regard to
>      the FMB enable/disable
>    - Assert that fmb_lock is held within zpci_fmb_enable_device() and
>      zpci_fmb_disable_device()
> 
> * Patch 2/4 (previously 1/3):
>    - Move the FMB enable logic into a static function zpci_fmb_do_enable()
>      to reduce code duplication between zpci_fmb_enable_device() and
>      zpci_fmb_reenable_device()
>    - Reword commit message to use the imperative voice more consistently
> 
> * Patch 3/4 (previously 2/3):
>    - Split the previous VFIO feature into a SET-only and a GET-only feature
>      for enabling/disabling and reading the FMB respectively
>    - Remove FMB definitions from the VFIO uAPI and instead treat it as an
>      opaque structure
> 
> * Patch 4/4 (previously 3/3):
>    - Clarify goto label name to reduce misunderstandings
> 
> v1 -> v2:
> * Patch 1/3:
>    - Address a possible race condition in zpci_reenable_device() caused by
>      calling zpci_fmb_reenable_device() without holding fmb_lock
>    - Assert that fmb_lock is held within zpci_fmb_reenable_device()
> 
> * Patch 3/3:
>    - Address a possible race condition in pci_perf_seq_write() caused by
>      consuming zdev->kzdev without holding kzdev_lock
> 
> Motivation
> ==========
> The firmware on s390x machines allows for tracking a variety of statistics
> relating to zPCI devices in a function measurement block (FMB). However,
> the kernel currently lacks a structured mechanism of sharing this
> information with userspace, beyond /sys/kernel/debug/pci/ID/statistics.
> This can lead to shortcomings when running a guest on KVM with PCI
> passthrough devices, as QEMU is unable to provide an accurate FMB snapshot
> to the guest.
> 
> Proposal
> ========
> We propose adding a new VFIO device feature to zPCI passthrough devices,
> allowing userspace programs to read the latest FMB snapshot as it is
> written by the firmware. We ensure that function measurement enablement is
> preserved across device resets on the host. Furthermore, we guard against
> host tampering with the FMB via sysfs when the zPCI device is in
> passthrough to protect the VM's state.
> 
> I'd appreciate some feedback on these patches.
> 
> Thanks in advance.
> 
> Omar Elghoul (4):
>    s390/pci: Hold fmb_lock when enabling or disabling PCI devices
>    s390/pci: Reuse FMB buffer and preserve state in device re-enablement
>    s390/pci: Fence FMB enable/disable via debugfs for passthrough devices
>    vfio-pci/zdev: Add VFIO FMB device features
> 
>   arch/s390/include/asm/pci.h      |  2 +
>   arch/s390/pci/pci.c              | 85 +++++++++++++++++++++++++-------
>   arch/s390/pci/pci_debug.c        | 11 ++++-
>   drivers/vfio/pci/vfio_pci_core.c |  4 ++
>   drivers/vfio/pci/vfio_pci_priv.h | 18 +++++++
>   drivers/vfio/pci/vfio_pci_zdev.c | 60 ++++++++++++++++++++++
>   include/uapi/linux/vfio.h        | 29 +++++++++++
>   7 files changed, 191 insertions(+), 18 deletions(-)
>