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

Lu Baolu
2020-07-08T02:58:18Z

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@linux.intel.com> 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);