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

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

> From: Tian, Kevin
> Sent: Friday, July 31, 2020 8:26 AM
> 
> > From: Alex Williamson <alex.williamson@redhat.com>
> > Sent: Friday, July 31, 2020 4:17 AM
> >
> > 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.
> 
> it's maintained in VT-d driver (include/linux/intel-iommu.h)
> 
> struct device_domain_info {
>         struct list_head link;  /* link to domain siblings */
>         struct list_head global; /* link to global list */
>         struct list_head table; /* link to pasid table */
>         struct list_head auxiliary_domains; /* auxiliary domains
>                                              * attached to this device
>                                              */
> 	...
> 
> >
> > > 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
> 
> It's the tradeoff between leaving breadcrumb in domain or in group.
> Today the domain has no knowledge of mdev. It just includes a list
> of physical devices which are attached to the domain (either due to
> the device is assigned in a whole or as the parent device of an assigned
> mdev). Then we have two choices. One is to save the mdev_dev info
> in device_domain_info and maintain a mapping between mdev_dev
> and related aux domain. The other is to record the domain info directly
> in group. Earlier we choose the latter one as it looks simpler. If you
> prefer to the former one, we can think more and have a try.
> 

Please skip this comment. I have a wrong understanding of the problem
and is discussing with Baolu. He will reply with our conclusion later.

Thanks
Kevin
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