RE: [PATCH v2 3/3] iommufd/device: Change iommufd_hw_pagetable_has_group to device centric

[Tian, Kevin]

> From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> Sent: Monday, January 30, 2023 6:22 PM
> 
> On Sun, Jan 29, 2023 at 02:38:55AM -0800, Nicolin Chen wrote:
> 
> > > > @@ -385,10 +372,8 @@ void iommufd_device_detach(struct
> > > > iommufd_device *idev)
> > > >       struct iommufd_hw_pagetable *hwpt = idev->hwpt;
> > > >
> > > >       mutex_lock(&hwpt->ioas->mutex);
> > > > -     mutex_lock(&hwpt->devices_lock);
> > > >       refcount_dec(hwpt->devices_users);
> > > > -     list_del(&idev->devices_item);
> > > > -     if (!iommufd_hw_pagetable_has_group(hwpt, idev->group)) {
> > > > +     if (iommufd_hw_pagetable_has_device(hwpt, idev->dev)) {
> > > >               if (refcount_read(hwpt->devices_users) == 1) {
> > > >                       iopt_table_remove_domain(&hwpt->ioas->iopt,
> > > >                                                hwpt->domain);
> > > > @@ -397,7 +382,6 @@ void iommufd_device_detach(struct
> iommufd_device
> > > > *idev)
> > > >               iommu_detach_group(hwpt->domain, idev->group);
> > > >       }
> > >
> > > emmm how do we track last device detach in a group? Here the first
> > > device detach already leads to group detach...
> >
> > Oh no. That's a bug. Thanks for catching it.
> >
> > We need an additional refcount somewhere to track the number of
> > attached devices in the iommu_group.
> 
> Wondering if we can let iommu_attach/detach_device handle this:
> 

that is the desired way to fully remove group awareness in iommufd.

but iirc there were some concerns on changing their semantics. But
I don't remember the detail now. Jason might know. also +Baolu/Robin.

otherwise as long as the group attach/detach continues to be used
then identifying last device in the group always needs some hack
within iommufd itself.