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

[Lu Baolu <baolu.lu@linux.intel.com>]

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
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