Re: [Intel-gfx] [PATCH v6 12/24] vfio/pci: Allow passing zero-length fd array in VFIO_DEVICE_PCI_HOT_RESET

[Alex Williamson <alex.williamson@redhat.com>]

On Wed,  8 Mar 2023 05:28:51 -0800
Yi Liu <yi.l.liu@intel.com> wrote:

> This is another method to issue PCI hot reset for the users that bounds
> device to a positive iommufd value. In such case, iommufd is a proof of
> device ownership. By passing a zero-length fd array, user indicates kernel
> to do ownership check with the bound iommufd. All the opened devices within
> the affected dev_set should have been bound to the same iommufd. This is
> 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.

Couldn't this same idea apply to containers?

I'm afraid this proposal reduces or eliminates the handshake we have
with userspace between VFIO_DEVICE_GET_PCI_HOT_RESET_INFO and
VFIO_DEVICE_PCI_HOT_RESET, which could promote userspace to ignore the
_INFO ioctl altogether, resulting in drivers that don't understand the
scope of the reset.  Is it worth it?  What do we really gain?

> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> index d80141969cd1..382d95455f89 100644
> --- a/include/uapi/linux/vfio.h
> +++ b/include/uapi/linux/vfio.h
> @@ -682,6 +682,11 @@ struct vfio_pci_hot_reset_info {
>   * The ownership can be proved by:
>   *   - An array of group fds
>   *   - An array of device 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_ctx.  This approach is only
> + * available for devices bound to positive iommufd.
>   *
>   * Return: 0 on success, -errno on failure.
>   */

There's no introspection that this feature is supported, is that why
containers are not considered?  ie. if the host supports vfio cdevs, it
necessarily must support vfio-pci hot reset w/ a zero-length array?
Thanks,

Alex