Re: [PATCH 6/9] vfio/pci: Clean up BAR zap and revocation

Alex Williamson
2026-05-01T23:19:24Z

On Thu, 16 Apr 2026 06:17:49 -0700
Matt Evans <mattev@meta.com> wrote:

> Previously, vfio_pci_zap_bars() (and the wrapper
> vfio_pci_zap_and_down_write_memory_lock()) calls were paired with
> calls of vfio_pci_dma_buf_move().
>
> This commit replaces them a unified new function,
> vfio_pci_zap_revoke_bars() containing both the vfio_pci_dma_buf_move()
> and the unmap_mapping_range(), making it harder for callers to omit
> one. It adds a wrapper, vfio_pci_lock_zap_revoke_bars(), which takes
> the write memory_lock before zapping, and adds a new
> vfio_pci_unrevoke_bars() for the re-enable path.
>
> However, as of "vfio/pci: Convert BAR mmap() to use a DMABUF" the
> unmap_mapping_range() to zap is entirely redundant for plain vfio-pci,
> since the DMABUFs used for BAR mappings already zap PTEs when the
> vfio_pci_dma_buf_move() occurs.
>
> One exception remains as a FIXME: in nvgrace-gpu, some BAR VMAs
> conditionally use custom vm_ops, which have not moved to be backed by
> DMABUFs. If these BARs are mmap()ed, the vdev enables the existing
> behaviour of unmap_mapping_range() for the device fd address space.

What's the plan here? Is this a temporary FIXME or a place to prove
that dmabuf for mmap works beyond the core use case?

>
> Signed-off-by: Matt Evans <mattev@meta.com>
> ---
> drivers/vfio/pci/nvgrace-gpu/main.c | 5 +++
> drivers/vfio/pci/vfio_pci_config.c | 30 ++++++--------
> drivers/vfio/pci/vfio_pci_core.c | 62 +++++++++++++++++++----------
> drivers/vfio/pci/vfio_pci_priv.h | 3 +-
> include/linux/vfio_pci_core.h | 1 +
> 5 files changed, 62 insertions(+), 39 deletions(-)
>
...
> @@ -1229,7 +1228,7 @@ static int vfio_pci_ioctl_reset(struct vfio_pci_core_device *vdev,
> if (!vdev->reset_works)
> return -EINVAL;
>
> - vfio_pci_zap_and_down_write_memory_lock(vdev);
> + vfio_pci_lock_zap_revoke_bars(vdev);
>
> /*
> * This function can be invoked while the power state is non-D0. If
> @@ -1242,10 +1241,9 @@ static int vfio_pci_ioctl_reset(struct vfio_pci_core_device *vdev,
> */
> vfio_pci_set_power_state(vdev, PCI_D0);
>
> - vfio_pci_dma_buf_move(vdev, true);

This seems subtle enough to be troublesome. I wonder if Leon didn't
intentionally place the dmabuf revoke after the device is in D0 to
allow the driver to interact with the device. I think the lock needs
to come before the power state change to avoid racing a user induced
state change. Thanks,

Alex