Re: [PATCH v4 0/4] vfio-pci/zdev: Improved zPCI Function Measurement Support

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

Polite ping :)

Thanks in advance.

On 6/12/26 2:10 PM, Omar Elghoul wrote:
> Hi,
> 
> This patch series improves support for function measurement for zPCI
> passthrough devices on s390x.
> 
> Changelog
> =========
> v3 -> v4:
> * Patch 2/3:
>    - 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: Preserve FMB state in device re-enablement
>    vfio-pci/zdev: Add VFIO FMB device features
>    s390/pci: Fence FMB enable/disable via sysfs for passthrough devices
> 
>   arch/s390/include/asm/pci.h      |   1 +
>   arch/s390/pci/pci.c              | 107 +++++++++++++++++++++++++------
>   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 |  55 ++++++++++++++++
>   include/uapi/linux/vfio.h        |  29 +++++++++
>   7 files changed, 206 insertions(+), 19 deletions(-)
>