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

[Lu Baolu]

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.
 */
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);

Best regards,
baolu