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

[Tian, Kevin]

> From: Alex Williamson <alex.williamson at redhat.com>
> Sent: Thursday, March 16, 2023 6:53 AM
> 
> On Wed,  8 Mar 2023 05:28:51 -0800
> Yi Liu <yi.l.liu at 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?

User is allowed to create multiple containers. Looks we don't have a way
to check whether multiple containers belong to the same user today.

> 
> 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?

Jason raised the concern whether GET_PCI_HOT_RESET_INFO is actually
useful today.

It's an interface on opened device. So the tiny difference is whether the
user knows the device is resettable when calling GET_INFO or later when
actually calling PCI_HOT_RESET.

and with this series we also allow reset on affected devices which are not
opened. Such dynamic cannot be reflected in static GET_INFO. More
suitable a try-and-fail style.


> 
> > 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,
> 

yes. It's more for users who knows that iommufd is used.