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

[Tian, Kevin]

> From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> Sent: Sunday, January 29, 2023 6:39 PM
> 
> On Sun, Jan 29, 2023 at 09:37:00AM +0000, Tian, Kevin wrote:
> > External email: Use caution opening links or attachments
> >
> >
> > > From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> > > Sent: Sunday, January 29, 2023 5:18 AM
> > >
> > > -static bool iommufd_hw_pagetable_has_group(struct
> > > iommufd_hw_pagetable *hwpt,
> > > -                                        struct iommu_group *group)
> > > +static bool iommufd_hw_pagetable_has_device(struct
> > > iommufd_hw_pagetable *hwpt,
> > > +                                         struct device *dev)
> > >  {
> > > -     struct iommufd_device *cur_dev;
> > > -
> > > -     list_for_each_entry(cur_dev, &hwpt->devices, devices_item)
> > > -             if (cur_dev->group == group)
> > > -                     return true;
> > > -     return false;
> > > +     /*
> > > +      * iommu_get_domain_for_dev() returns an iommu_group->domain
> > > ptr, if it
> > > +      * is the same domain as the hwpt->domain, it means that this hwpt
> > > has
> > > +      * the iommu_group/device.
> > > +      */
> > > +     return hwpt->domain == iommu_get_domain_for_dev(dev);
> > >  }
> >
> > Here we could have three scenarios:
> >
> > 1) the device is attached to blocked domain;
> > 2) the device is attached to hwpt->domain;
> > 3) the device is attached to another hwpt->domain;
> >
> > if this function returns false then iommufd_device_do_attach() will attach
> > the device to the specified hwpt. But then it's wrong for 3).
> >
> > Has 3) been denied in earlier path? If yes at least a WARN_ON for
> > case 3) makes sense here.
> 
> The case #3 means the device is already attached to some other
> domain? Then vfio_iommufd_physical_attach_ioas returns -EBUSY
> at the sanity of vdev->iommufd_attached. And the case #3 feels
> like a domain replacement use case to me. So probably not that
> necessary to add a wARN_ON?
> 

You are right. I thought about the cdev case where the device is
not attached in vfio but has a valid domain due to attach status
of other devices in the group. But even in this case it's user's
responsibility to not break group boundary. So yes it's just a
domain replacement and WARN_ON is not required.