[Intel-gfx] [PATCH v9 06/25] kvm/vfio: Accept vfio device file from userspace

[Liu, Yi L]

> From: Alex Williamson <alex.williamson at redhat.com>
> Sent: Friday, April 7, 2023 2:58 AM
> > >
> > > You don't say anything about potential restriction, ie. what if the user calls
> > > KVM_DEV_VFIO_FILE with device fds while it has been using legacy
> container/group
> > > API?
> >
> > legacy container/group path cannot do it as the below enhancement.
> > User needs to call KVM_DEV_VFIO_FILE before open devices, so this
> > should happen before _GET_DEVICE_FD. So the legacy path can never
> > pass device fds in KVM_DEV_VFIO_FILE.
> >
> >
> https://lore.kernel.org/kvm/20230327102059.333d6976.alex.williamson@redhat.com
> /#t
> 
> Wait, are you suggesting that a comment in the documentation suggesting
> a usage policy somehow provides enforcement of that ordering??  That's
> not how this works.  Thanks,

I don't know if there is a good way to enforce this order in the code. The
vfio_device->kvm pointer is optional. If it is NULL, vfio just ignores it.
So vfio doesn't have a good way to tell if the order requirement is met or
not. Perhaps just trigger NULL pointer dereference when kvm pointer is used
in the device drivers like kvmgt if this order is not met.

So that's why I come up to document it here. The applications uses kvm
should know this and follow this otherwise it may encounter error.

Do you have other suggestions for it? This order should be a generic
requirement. is it? group path also needs to follow it to make the mdev
driver that refers kvm pointer to be workable.

Thanks,
Yi Liu

> > > > -The GROUP_ADD operation above should be invoked prior to accessing the
> > > > +The FILE/GROUP_ADD operation above should be invoked prior to accessing the
> > > >  device file descriptor via VFIO_GROUP_GET_DEVICE_FD in order to support
> > > >  drivers which require a kvm pointer to be set in their .open_device()
> > > > -callback.
> > > > +callback.  It is the same for device file descriptor via character device
> > > > +open which gets device access via VFIO_DEVICE_BIND_IOMMUFD.  For such file
> > > > +descriptors, FILE_ADD should be invoked before
> VFIO_DEVICE_BIND_IOMMUFD
> > > > +to support the drivers mentioned in prior sentence as well.
> >
> > just as here. This means device fds can only be passed with KVM_DEV_VFIO_FILE
> > in the cdev path.
> >
> > Regards,
> > Yi Liu