Re: [PATCH v3 3/4] iommu: Add iommu_aux_get_domain_for_dev()

Lu Baolu
2020-07-31T06:30:13Z

Hi Alex,

On 2020/7/30 4:25, Alex Williamson wrote:
> On Tue, 14 Jul 2020 13:57:02 +0800
> Lu Baolu<baolu.lu@linux.intel.com> wrote:
>
>> The device driver needs an API to get its aux-domain. A typical usage
>> scenario is:
>>
>> unsigned long pasid;
>> struct iommu_domain *domain;
>> struct device *dev = mdev_dev(mdev);
>> struct device *iommu_device = vfio_mdev_get_iommu_device(dev);
>>
>> domain = iommu_aux_get_domain_for_dev(dev);
>> if (!domain)
>> return -ENODEV;
>>
>> pasid = iommu_aux_get_pasid(domain, iommu_device);
>> if (pasid <= 0)
>> return -EINVAL;
>>
>> /* Program the device context */
>> ....
>>
>> This adds an API for such use case.
>>
>> Suggested-by: Alex Williamson<alex.williamson@redhat.com>
>> Signed-off-by: Lu Baolu<baolu.lu@linux.intel.com>
>> ---
>> drivers/iommu/iommu.c | 18 ++++++++++++++++++
>> include/linux/iommu.h | 7 +++++++
>> 2 files changed, 25 insertions(+)
>>
>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>> index cad5a19ebf22..434bf42b6b9b 100644
>> --- a/drivers/iommu/iommu.c
>> +++ b/drivers/iommu/iommu.c
>> @@ -2817,6 +2817,24 @@ void iommu_aux_detach_group(struct iommu_domain *domain,
>> }
>> EXPORT_SYMBOL_GPL(iommu_aux_detach_group);
>>
>> +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;
> Why wouldn't the aux domain flag be on the domain itself rather than
> the group? Then if we wanted sanity checking in patch 1/ we'd only
> need to test the flag on the object we're provided.

Agreed. Given that a group may contain both non-aux and aux devices,
adding such flag in iommu_group doesn't make sense.

>
> If we had such a flag, we could create an iommu_domain_is_aux()
> function and then simply use iommu_get_domain_for_dev() and test that
> it's an aux domain in the example use case. It seems like that would
> resolve the jump from a domain to an aux-domain just as well as adding
> this separate iommu_aux_get_domain_for_dev() interface. The is_aux
> test might also be useful in other cases too.

Let's rehearsal our use case.

unsigned long pasid;
struct iommu_domain *domain;
struct device *dev = mdev_dev(mdev);
struct device *iommu_device = vfio_mdev_get_iommu_device(dev);

[1] domain = iommu_get_domain_for_dev(dev);
if (!domain)
return -ENODEV;

[2] pasid = iommu_aux_get_pasid(domain, iommu_device);
if (pasid <= 0)
return -EINVAL;

/* Program the device context */
....

The reason why I add this iommu_aux_get_domain_for_dev() is that we need
to make sure the domain got at [1] is valid to be used at [2].

https://lore.kernel.org/linux-iommu/20200707150408.474d81f1@x1.home/

When calling into iommu_aux_get_pasid(), the iommu driver should make
sure that @domain is a valid aux-domain for @iommu_device. Hence, for
our use case, it seems that there's no need for a is_aux_domain() api.

Anyway, I'm not against adding a new is_aux_domain() api if there's a
need elsewhere.

Best regards,
baolu