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

[Lu Baolu]

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@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>  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@xxxxxxxxxx>
> > > > Signed-off-by: Lu Baolu<baolu.lu@xxxxxxxxxxxxxxx>
> > > > ---
> > > >    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@xxxxxxx/
>
> 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