RE: [PATCH v3 05/12] vfio/pci: Allow passing zero-length fd array in VFIO_DEVICE_PCI_HOT_RESET

Liu, Yi L
2023-04-05T07:55:19Z

> From: Alex Williamson <alex.williamson@redhat.com>
> Sent: Wednesday, April 5, 2023 4:19 AM
>
> On Sat, 1 Apr 2023 07:44:22 -0700
> Yi Liu <yi.l.liu@intel.com> wrote:
>
> > as an alternative method for ownership check when iommufd is used. In
> > this case all opened devices in the affected dev_set are verified to
> > be bound to a same valid iommufd value to allow reset. It's simpler
> > and faster as user does not need to pass a set of fds and kernel no
> > need to search the device within the given fds.
> >
> > a device in noiommu mode doesn't have a valid iommufd, so this method
> > should not be used in a dev_set which contains multiple devices and one
> > of them is in noiommu. The only allowed noiommu scenario is that the
> > calling device is noiommu and it's in a singleton dev_set.
> >
> > Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> > Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> > Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> > Tested-by: Yanting Jiang <yanting.jiang@intel.com>
> > Signed-off-by: Yi Liu <yi.l.liu@intel.com>
> > ---
> > drivers/vfio/pci/vfio_pci_core.c | 42 +++++++++++++++++++++++++++-----
> > include/uapi/linux/vfio.h | 9 ++++++-
> > 2 files changed, 44 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> > index 3696b8e58445..b68fcba67a4b 100644
> > --- a/drivers/vfio/pci/vfio_pci_core.c
> > +++ b/drivers/vfio/pci/vfio_pci_core.c
> > @@ -180,7 +180,8 @@ static void vfio_pci_probe_mmaps(struct
> vfio_pci_core_device *vdev)
> > struct vfio_pci_group_info;
> > static void vfio_pci_dev_set_try_reset(struct vfio_device_set *dev_set);
> > static int vfio_pci_dev_set_hot_reset(struct vfio_device_set *dev_set,
> > - struct vfio_pci_group_info *groups);
> > + struct vfio_pci_group_info *groups,
> > + struct iommufd_ctx *iommufd_ctx);
> >
> > /*
> > * INTx masking requires the ability to disable INTx signaling via PCI_COMMAND
> > @@ -1277,7 +1278,7 @@ vfio_pci_ioctl_pci_hot_reset_groups(struct
> vfio_pci_core_device *vdev,
> > return ret;
> >
> > /* Somewhere between 1 and count is OK */
> > - if (!hdr->count || hdr->count > count)
> > + if (hdr->count > count)
> > return -EINVAL;
> >
> > group_fds = kcalloc(hdr->count, sizeof(*group_fds), GFP_KERNEL);
> > @@ -1326,7 +1327,7 @@ vfio_pci_ioctl_pci_hot_reset_groups(struct
> vfio_pci_core_device *vdev,
> > info.count = hdr->count;
> > info.files = files;
> >
> > - ret = vfio_pci_dev_set_hot_reset(vdev->vdev.dev_set, &info);
> > + ret = vfio_pci_dev_set_hot_reset(vdev->vdev.dev_set, &info, NULL);
> >
> > hot_reset_release:
> > for (file_idx--; file_idx >= 0; file_idx--)
> > @@ -1341,6 +1342,7 @@ static int vfio_pci_ioctl_pci_hot_reset(struct
> vfio_pci_core_device *vdev,
> > {
> > unsigned long minsz = offsetofend(struct vfio_pci_hot_reset, count);
> > struct vfio_pci_hot_reset hdr;
> > + struct iommufd_ctx *iommufd;
> > bool slot = false;
> >
> > if (copy_from_user(&hdr, arg, minsz))
> > @@ -1355,7 +1357,12 @@ static int vfio_pci_ioctl_pci_hot_reset(struct
> vfio_pci_core_device *vdev,
> > else if (pci_probe_reset_bus(vdev->pdev->bus))
> > return -ENODEV;
> >
> > - return vfio_pci_ioctl_pci_hot_reset_groups(vdev, &hdr, slot, arg);
> > + if (hdr.count)
> > + return vfio_pci_ioctl_pci_hot_reset_groups(vdev, &hdr, slot, arg);
> > +
> > + iommufd = vfio_iommufd_physical_ictx(&vdev->vdev);
> > +
> > + return vfio_pci_dev_set_hot_reset(vdev->vdev.dev_set, NULL, iommufd);
> > }
> >
> > static int vfio_pci_ioctl_ioeventfd(struct vfio_pci_core_device *vdev,
> > @@ -2327,6 +2334,9 @@ static bool vfio_dev_in_groups(struct
> vfio_pci_core_device *vdev,
> > {
> > unsigned int i;
> >
> > + if (!groups)
> > + return false;
> > +
> > for (i = 0; i < groups->count; i++)
> > if (vfio_file_has_dev(groups->files[i], &vdev->vdev))
> > return true;
> > @@ -2402,13 +2412,25 @@ static int vfio_pci_dev_set_pm_runtime_get(struct
> vfio_device_set *dev_set)
> > return ret;
> > }
> >
> > +static bool vfio_dev_in_iommufd_ctx(struct vfio_pci_core_device *vdev,
> > + struct iommufd_ctx *iommufd_ctx)
> > +{
> > + struct iommufd_ctx *iommufd = vfio_iommufd_physical_ictx(&vdev->vdev);
> > +
> > + if (!iommufd)
> > + return false;
> > +
> > + return iommufd == iommufd_ctx;
> > +}
> > +
> > /*
> > * We need to get memory_lock for each device, but devices can share mmap_lock,
> > * therefore we need to zap and hold the vma_lock for each device, and only then
> > * get each memory_lock.
> > */
> > static int vfio_pci_dev_set_hot_reset(struct vfio_device_set *dev_set,
> > - struct vfio_pci_group_info *groups)
> > + struct vfio_pci_group_info *groups,
> > + struct iommufd_ctx *iommufd_ctx)
> > {
> > struct vfio_pci_core_device *cur_mem;
> > struct vfio_pci_core_device *cur_vma;
> > @@ -2448,9 +2470,17 @@ static int vfio_pci_dev_set_hot_reset(struct
> vfio_device_set *dev_set,
> > *
> > * Otherwise all opened devices in the dev_set must be
> > * contained by the set of groups provided by the user.
> > + *
> > + * If user provides a zero-length array, then all the
> > + * opened devices must be bound to a same iommufd_ctx.
> > + *
> > + * If all above checks are failed, reset is allowed only if
> > + * the calling device is in a singleton dev_set.
> > */
> > if (cur_vma->vdev.open_count &&
> > - !vfio_dev_in_groups(cur_vma, groups)) {
> > + !vfio_dev_in_groups(cur_vma, groups) &&
> > + !vfio_dev_in_iommufd_ctx(cur_vma, iommufd_ctx) &&
> > + (dev_set->device_count > 1)) {
>
> This last condition looks buggy to me, we need all conditions to be
> true to generate an error here, which means that for a singleton
> dev_set, it doesn't matter what group fds are passed, if any, or whether
> the iommufd context matches. I think in fact this means that the empty
> array path is equally available for group use cases with a singleton
> dev_set, but we don't enable it for multiple device dev_sets like we do
> iommufd.

you are right. The last condition allows the empty-fd array path to
work for the group use case if the dev_set happens to be a singleton.

>
> You pointed out a previous issue with hot-reset info and no-iommu where
> if other affected devices are not bound to vfio-pci the info ioctl
> returns error. That's handled in the hot-reset ioctl by the fact that
> all affected devices must be in the dev_set and therefore bound to
> vfio-pci drivers.

yes, hot-reset ioctl requires all affected devices listed in the dev_set.
So for the case there are devices not bound to vfio yet, hot-reset ioctl
just fails. If all affected devices are in the dev_set, they will have a
fake group allocated by vfio. So the info ioctl won't fail.

> So it seems to me that aside from the spurious error
> because we can't report an iommu group when none exists, and didn't
> spot it to invent an invalid group for debugging, hot-reset otherwise
> works with no-iommu just like it does for iommu backed devices. We
> don't currently require singleton no-iommu dev_sets afaict.

yes. the requirement for hot-reset is the same between no-iommu and
the iommufd backed devices.

> I'll also note that if the dev_set is singleton, this suggests that
> pci_reset_function() can make use of bus reset, so a hot-reset is
> accessible via VFIO_DEVICE_RESET if the appropriate reset method is
> selected.

yes. so does it mean not necessary to allow singleton dev_set support
in hot-reset ioctl? If user uses hot-reset, it should because of unable to
use VFIO_DEVICE_RESET, is it?

>
> Therefore, I think as written, the singleton dev_set hot-reset is
> enabled for iommufd and (unintentionally?) for the group path, while
> also negating a requirement for a group fd or that a provided group fd
> actually matches the device in this latter case. The null-array
> approach is not however extended to groups for more general use.
> Additionally, limiting no-iommu hot-reset to singleton dev_sets
> provides only a marginal functional difference vs VFIO_DEVICE_RESET.

I think the singletion dev_set hot-reset is for iommufd (or more accurately
for the noiommu case in cdev path).

> Thanks,
>
> Alex
>
> > ret = -EINVAL;
> > goto err_undo;
> > }
> > diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> > index f96e5689cffc..17aa5d09db41 100644
> > --- a/include/uapi/linux/vfio.h
> > +++ b/include/uapi/linux/vfio.h
> > @@ -679,7 +679,14 @@ struct vfio_pci_hot_reset_info {
> > * the calling user must ensure all affected devices, if opened, are
> > * owned by itself.
> > *
> > - * The ownership is proved by an array of group fds.
> > + * The ownership can be proved by:
> > + * - An array of group fds
> > + * - A zero-length array
> > + *
> > + * In the last case all affected devices which are opened by this user
> > + * must have been bound to a same iommufd. If the calling device is in
> > + * noiommu mode (no valid iommufd) then it can be reset only if the reset
> > + * doesn't affect other devices.
> > *
> > * Return: 0 on success, -errno on failure.
> > */