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

Alex Williamson
2020-07-31T18:14:30Z

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. Thanks,

Alex