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

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

Hi Alex,

On 8/1/20 2:14 AM, Alex Williamson wrote:
> On Fri, 31 Jul 2020 14:30:03 +0800
> Lu Baolu <baolu.lu@linux.intel.com> wrote:
> 
>> 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/
> 
> Yep, I thought that was a bit of a leap in logic.
> 
>> 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.
> 
> I think it could work either way, we could have an
> iommu_get_aux_domain_for_dev() which returns NULL if the domain is not
> an aux domain, or we could use iommu_get_domain_for_dev() and the
> caller could test the domain with iommu_is_aux_domain() if they need to
> confirm if it's an aux domain.  The former could even be written using
> the latter, a wrapper of iommu_get_domain_for_dev() that checks aux
> property before returning.

Okay. So iommu_get_domain_for_dev() and iommu_is_aux_domain() are what
we wanted. The iommu_get_domain_for_dev() could be a simple wrapper of
them.

Thanks a lot for the guide. I will implement a new version according to
the feedbacks.

Best regards,
baolu
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