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

[Tian, Kevin]

> From: Yi Liu
> Sent: Wednesday, March 8, 2023 9:29 PM
> 
> 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.
> 
> Suggested-by: Jason Gunthorpe <jgg at nvidia.com>
> Signed-off-by: Yi Liu <yi.l.liu at intel.com>

I think you also need a s-o-b from Jason since he wrote most of the
code here.

> +struct iommufd_ctx *vfio_iommufd_physical_ctx(struct vfio_device *vdev)
> +{
> +	/* Only serve for physical device */
> +	if (!vdev->iommufd_device)
> +		return NULL;

pointless comment

> --- 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.

As we chatted before I still think the last sentence is pointless. If a device
is bound to negative iommufd value (i.e. noiommu) it doesn't have a
valid iommufd_ctx so won't meet "bound to a same iommufd_ctx".