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

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 29 Jul 2020 23:49:20 +0000
"Tian, Kevin" <kevin.tian@intel.com> wrote:

> > 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

Is this the IOMMU layer or the VT-d driver?  I don't see any notion of
managing domains relative to a parent in the IOMMU layer.  Please point
to something more specific if I'm wrong here.

> 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. 

Not the parent device of the mdev, but the mdev_dev(mdev) device.
Isn't that what this series is enabling, being able to return the
domain from the group that contains the mdev_dev?  We shouldn't need to
leave breadcrumbs on the group to know about the domain, the domain
itself should be the source of knowledge, or provide a mechanism/ops to
learn that knowledge.  Thanks,

Alex


> > 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)  
> > >  {  
> 

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