Re: [PATCH v2 1/2] iommu: iommu_aux_at(de)tach_device() extension

[Lu Baolu]

Hi Alex,

On 7/9/20 3:07 AM, Alex Williamson wrote:
> On Wed, 8 Jul 2020 10:53:12 +0800
> Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx> wrote:
>
> > Hi Alex,
> >
> > Thanks a lot for your comments. Please check my reply inline.
> >
> > On 7/8/20 5:04 AM, Alex Williamson wrote:
> > > On Tue,  7 Jul 2020 09:39:56 +0800
> > > Lu Baolu<baolu.lu@xxxxxxxxxxxxxxx>  wrote:
> > >    
> > > > The hardware assistant vfio mediated device is a use case of iommu
> > > > aux-domain. The interactions between vfio/mdev and iommu during mdev
> > > > creation and passthr are:
> > > >
> > > > - Create a group for mdev with iommu_group_alloc();
> > > > - Add the device to the group with
> > > >           group = iommu_group_alloc();
> > > >           if (IS_ERR(group))
> > > >                   return PTR_ERR(group);
> > > >
> > > >           ret = iommu_group_add_device(group, &mdev->dev);
> > > >           if (!ret)
> > > >                   dev_info(&mdev->dev, "MDEV: group_id = %d\n",
> > > >                            iommu_group_id(group));
> > > > - Allocate an aux-domain
> > > >           iommu_domain_alloc()
> > > > - Attach the aux-domain to the physical device from which the mdev is
> > > >     created.
> > > >           iommu_aux_attach_device()
> > > >
> > > > In the whole process, an iommu group was allocated for the mdev and an
> > > > iommu domain was attached to the group, but the group->domain leaves
> > > > NULL. As the result, iommu_get_domain_for_dev() doesn't work anymore.
> > > >
> > > > The iommu_get_domain_for_dev() is a necessary interface for device
> > > > drivers that want to support aux-domain. For example,
> > > >
> > > >           struct iommu_domain *domain;
> > > >           struct device *dev = mdev_dev(mdev);
> > > >           unsigned long pasid;
> > > >
> > > >           domain = iommu_get_domain_for_dev(dev);
> > > >           if (!domain)
> > > >                   return -ENODEV;
> > > >
> > > >           pasid = iommu_aux_get_pasid(domain, dev->parent);
> > > How did we know this was an aux domain? ie. How did we know we could
> > > use it with iommu_aux_get_pasid()?
> >
> > Yes. It's a bit confusing if iommu_get_domain_for_dev() is reused here
> > for aux-domain.
> >
> > > Why did we assume the parent device is the iommu device for the aux
> > > domain?  Should that level of detail be already known by the aux domain?
> > >
> > > Nits - The iomu device of an mdev device is found via
> > > mdev_get_iommu_device(dev), it should not be assumed to be the parent.
> > > The parent of an mdev device is found via mdev_parent_dev(mdev).
> >
> > My bad. The driver should use mdev_get_iommu_device() instead.
> >
> > > The leaps in logic here make me wonder if we should instead be exposing
> > > more of an aux domain API rather than blurring the differences between
> > > these domains.  Thanks,
> >
> > How about add below API?
> >
> > /**
> >    * iommu_aux_get_domain_for_dev - get aux domain for a device
> >    * @dev: the accessory device
> >    *
> >    * The caller should pass a valid @dev to iommu_aux_attach_device() before
> >    * calling this api. Return an attached aux-domain, or NULL otherwise.
>
> That's not necessarily the caller's responsibility, that might happen
> elsewhere, this function simply returns an aux domain for the device if
> it's attached to one.

Yes. Fair enough. This piece of comments will be removed.

> >    */
> > struct iommu_domain *iommu_aux_get_domain_for_dev(struct device *dev)
> > {
> >           struct iommu_domain *domain = NULL;
> >           struct iommu_group *group;
> >
> >           group = iommu_group_get(dev);
> >           if (!group)
> >                   return NULL;
> >
> >           if (group->aux_domain_attached)
> >                   domain = group->domain;
> >
> >           iommu_group_put(group);
> >
> >           return domain;
> > }
> > EXPORT_SYMBOL_GPL(iommu_aux_get_domain_for_dev);
>
> For your example use case, this seems more clear to me.  Thanks,

Okay, thank you!

> Alex

Best regards,
baolu