RE: [PATCH] drm/i915/gvt: Add missing vfio_unregister_group_dev() call

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Wang, Zhi A <zhi.a.wang@intel.com>
> Sent: Wednesday, October 19, 2022 5:41 PM
> 
> On 10/6/22 18:31, Alex Williamson wrote:
> > On Thu, 6 Oct 2022 08:37:09 -0300
> > Jason Gunthorpe <jgg@nvidia.com> wrote:
> >
> >> On Wed, Oct 05, 2022 at 04:03:56PM -0600, Alex Williamson wrote:
> >>> We can't have a .remove callback that does nothing, this breaks
> >>> removing the device while it's in use.  Once we have the
> >>> vfio_unregister_group_dev() fix below, we'll block until the device is
> >>> unused, at which point vgpu->attached becomes false.  Unless I'm
> >>> missing something, I think we should also follow-up with a patch to
> >>> remove that bogus warn-on branch, right?  Thanks,
> >>
> >> Yes, looks right to me.
> >>
> >> I question all the logical arround attached, where is the locking?
> >
> > Zhenyu, Zhi, Kevin,
> >
> > Could someone please take a look at use of vgpu->attached in the GVT-g
> > driver?  It's use in intel_vgpu_remove() is bogus, the .release
> > callback needs to use vfio_unregister_group_dev() to wait for the
> > device to be unused.  The WARN_ON/return here breaks all future use of
> > the device.  I assume @attached has something to do with the page table
> > interface with KVM, but it all looks racy anyway.
> >
> Thanks for pointing this out.
> 
> It was introduced in the GVT-g refactor patch series and Christoph might
> not want to touch the vgpu->released while he needed a new state.
> 
> I dig it a bit. vgpu->attached would be used for preventing multiple open
> on a single vGPU and indicate the kvm_get_kvm() has been done.

vfio core already ensures that .open_device() is called only once:

vfio_device_open()
{
	...
	mutex_lock(&device->dev_set->lock);
	device->open_count++;
	if (device->open_count == 1) {
		...
		if (device->ops->open_device) {
			ret = device->ops->open_device(device);
			...
}

> vgpu->released was to prevent the release before close, which is now
> handled by the vfio_device_*.
> 
> What I would like to do are:
> 1) Remove the vgpu->released. 2) Use alock to protect vgpu->attached.
> 
> After those were solved, the WARN_ON/return in the intel_vgpu_remove()
> should be safely removed as the .release will be called after .close_device
> deceases the vfio_device->refcnt to zero.
> 
> Thanks,
> Zhi.
> 
> > Also, whatever purpose vgpu->released served looks unnecessary now.
> > Thanks,
> >
> > Alex
> >