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

[Baolu Lu]

On 2023/2/1 11:07, Tian, Kevin wrote:
> > 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.

I have tried to solve this problem.

https://lore.kernel.org/linux-iommu/20220106022053.2406748-1-baolu.lu@xxxxxxxxxxxxxxx/

I may need to review the original discussion to see if I can update a
new version.

Best regards,
baolu