linux-kernel - Re: [PATCH v3 3/4] iommu: Add iommu_aux_get_domain_for

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Date:   Fri, 31 Jul 2020 12:14:18 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Cornelia Huck <cohuck@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v3 3/4] iommu: Add iommu_aux_get_domain_for_dev()

On Fri, 31 Jul 2020 14:30:03 +0800
Lu Baolu <baolu.lu@...ux.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@...ux.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@...hat.com>
> >> Signed-off-by: Lu Baolu<baolu.lu@...ux.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