Re: [PATCH v3 4/4] vfio/type1: Use iommu_aux_at(de)tach_group() APIs

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

On Thu, 30 Jul 2020 10:41:32 +0800
Lu Baolu <baolu.lu@linux.intel.com> wrote:

> Hi Alex,
> 
> On 7/30/20 4:32 AM, Alex Williamson wrote:
> > On Tue, 14 Jul 2020 13:57:03 +0800
> > Lu Baolu <baolu.lu@linux.intel.com> wrote:
> >   
> >> Replace iommu_aux_at(de)tach_device() with iommu_aux_at(de)tach_group().
> >> It also saves the IOMMU_DEV_FEAT_AUX-capable physcail device in the
> >> vfio_group data structure so that it could be reused in other places.
> >>
> >> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
> >> ---
> >>   drivers/vfio/vfio_iommu_type1.c | 44 ++++++---------------------------
> >>   1 file changed, 7 insertions(+), 37 deletions(-)
> >>
> >> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> >> index 5e556ac9102a..f8812e68de77 100644
> >> --- a/drivers/vfio/vfio_iommu_type1.c
> >> +++ b/drivers/vfio/vfio_iommu_type1.c
> >> @@ -100,6 +100,7 @@ struct vfio_dma {
> >>   struct vfio_group {
> >>   	struct iommu_group	*iommu_group;
> >>   	struct list_head	next;
> >> +	struct device		*iommu_device;
> >>   	bool			mdev_group;	/* An mdev group */
> >>   	bool			pinned_page_dirty_scope;
> >>   };
> >> @@ -1627,45 +1628,13 @@ static struct device *vfio_mdev_get_iommu_device(struct device *dev)
> >>   	return NULL;
> >>   }
> >>   
> >> -static int vfio_mdev_attach_domain(struct device *dev, void *data)
> >> -{
> >> -	struct iommu_domain *domain = data;
> >> -	struct device *iommu_device;
> >> -
> >> -	iommu_device = vfio_mdev_get_iommu_device(dev);
> >> -	if (iommu_device) {
> >> -		if (iommu_dev_feature_enabled(iommu_device, IOMMU_DEV_FEAT_AUX))
> >> -			return iommu_aux_attach_device(domain, iommu_device);
> >> -		else
> >> -			return iommu_attach_device(domain, iommu_device);
> >> -	}
> >> -
> >> -	return -EINVAL;
> >> -}
> >> -
> >> -static int vfio_mdev_detach_domain(struct device *dev, void *data)
> >> -{
> >> -	struct iommu_domain *domain = data;
> >> -	struct device *iommu_device;
> >> -
> >> -	iommu_device = vfio_mdev_get_iommu_device(dev);
> >> -	if (iommu_device) {
> >> -		if (iommu_dev_feature_enabled(iommu_device, IOMMU_DEV_FEAT_AUX))
> >> -			iommu_aux_detach_device(domain, iommu_device);
> >> -		else
> >> -			iommu_detach_device(domain, iommu_device);
> >> -	}
> >> -
> >> -	return 0;
> >> -}
> >> -
> >>   static int vfio_iommu_attach_group(struct vfio_domain *domain,
> >>   				   struct vfio_group *group)
> >>   {
> >>   	if (group->mdev_group)
> >> -		return iommu_group_for_each_dev(group->iommu_group,
> >> -						domain->domain,
> >> -						vfio_mdev_attach_domain);
> >> +		return iommu_aux_attach_group(domain->domain,
> >> +					      group->iommu_group,
> >> +					      group->iommu_device);  
> > 
> > No, we previously iterated all devices in the group and used the aux
> > interface only when we have an iommu_device supporting aux.  If we
> > simply assume an mdev group only uses an aux domain we break existing
> > users, ex. SR-IOV VF backed mdevs.  Thanks,  
> 
> Oh, yes. Sorry! I didn't consider the physical device backed mdevs
> cases.
> 
> Looked into this part of code, it seems that there's a lock issue here.
> The group->mutex is held in iommu_group_for_each_dev() and will be
> acquired again in iommu_attach_device().

These are two different groups.  We walk the devices in the mdev's
group with iommu_group_for_each_dev(), holding the mdev's group lock,
but we call iommu_attach_device() with iommu_device, which results in
acquiring the lock for the iommu_device's group.

> How about making it like:
> 
> static int vfio_iommu_attach_group(struct vfio_domain *domain,
>                                     struct vfio_group *group)
> {
>          if (group->mdev_group) {
>                  struct device *iommu_device = group->iommu_device;
> 
>                  if (WARN_ON(!iommu_device))
>                          return -EINVAL;
> 
>                  if (iommu_dev_feature_enabled(iommu_device, 
> IOMMU_DEV_FEAT_AUX))
>                          return iommu_aux_attach_device(domain->domain, 
> iommu_device);
>                  else
>                          return iommu_attach_device(domain->domain, 
> iommu_device);
>          } else {
>                  return iommu_attach_group(domain->domain, 
> group->iommu_group);
>          }
> }
> 
> The caller (vfio_iommu_type1_attach_group) has guaranteed that all mdevs
> in an iommu group should be derived from a same physical device.

Have we?  iommu_attach_device() will fail if the group is not
singleton, but that's just encouraging us to use the _attach_group()
interface where the _attach_device() interface is relegated to special
cases.  Ideally we'd get out of those special cases and create an
_attach_group() for aux that doesn't further promote these notions.

> Any thoughts?

See my reply to Kevin, I'm thinking we need to provide a callback that
can enlighten the IOMMU layer to be able to do _attach_group() with
aux or separate IOMMU backed devices.  Thanks,

Alex

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