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

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Alex Williamson <alex.williamson@redhat.com>
> Sent: Thursday, July 30, 2020 4:25 AM
> 
> 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.
> 
> 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

IOMMU layer manages domains per parent device. Here given a
dev (of mdev), we need a way to find its associated domain under its
parent device. And we cannot simply use iommu_get_domain_for_dev
on the parent device of the mdev, as it will give us the primary domain
of parent device. 

Thanks
Kevin

> 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.  Thanks,
> 
> Alex
> 
> > +
> > +	iommu_group_put(group);
> > +
> > +	return domain;
> > +}
> > +EXPORT_SYMBOL_GPL(iommu_aux_get_domain_for_dev);
> > +
> >  /**
> >   * iommu_sva_bind_device() - Bind a process address space to a device
> >   * @dev: the device
> > diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> > index 9506551139ab..cda6cef7579e 100644
> > --- a/include/linux/iommu.h
> > +++ b/include/linux/iommu.h
> > @@ -639,6 +639,7 @@ int iommu_aux_attach_group(struct
> iommu_domain *domain,
> >  			   struct iommu_group *group, struct device *dev);
> >  void iommu_aux_detach_group(struct iommu_domain *domain,
> >  			   struct iommu_group *group, struct device *dev);
> > +struct iommu_domain *iommu_aux_get_domain_for_dev(struct device
> *dev);
> >
> >  struct iommu_sva *iommu_sva_bind_device(struct device *dev,
> >  					struct mm_struct *mm,
> > @@ -1040,6 +1041,12 @@ iommu_aux_detach_group(struct
> iommu_domain *domain,
> >  {
> >  }
> >
> > +static inline struct iommu_domain *
> > +iommu_aux_get_domain_for_dev(struct device *dev)
> > +{
> > +	return NULL;
> > +}
> > +
> >  static inline struct iommu_sva *
> >  iommu_sva_bind_device(struct device *dev, struct mm_struct *mm, void
> *drvdata)
> >  {