vfio: Use IOMMU_CAP_ENFORCE_CACHE_COHERENCY for vfio_file_enforced_coherent()

iommufd doesn't establish the iommu_domains until after the device FD is
opened, even if the container has been set. This design is part of moving
away from the group centric iommu APIs.

This is fine, except that the normal sequence of establishing the kvm
wbinvd won't work:

   group = open("/dev/vfio/XX")
   ioctl(group, VFIO_GROUP_SET_CONTAINER)
   ioctl(kvm, KVM_DEV_VFIO_GROUP_ADD)
   ioctl(group, VFIO_GROUP_GET_DEVICE_FD)

As the domains don't start existing until GET_DEVICE_FD. Further,
GET_DEVICE_FD requires that KVM_DEV_VFIO_GROUP_ADD already be done as that
is what sets the group->kvm and thus device->kvm for the driver to use
during open.

Now that we have device centric cap ops and the new
IOMMU_CAP_ENFORCE_CACHE_COHERENCY we know what the iommu_domain will be
capable of without having to create it. Use this to compute
vfio_file_enforced_coherent() and resolve the ordering problems.

VFIO always tries to upgrade domains to enforce cache coherency, it never
attaches a device that supports enforce cache coherency to a less capable
domain, so the cap test is a sufficient proxy for the ultimate
outcome. iommufd also ensures that devices that set the cap will be
connected to enforcing domains.

Link: https://lore.kernel.org/r/4-v4-42cd2eb0e3eb+335a-vfio_iommufd_jgg@nvidia.com


Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Yi Liu <yi.l.liu@intel.com>
Tested-by: Lixiao Yang <lixiao.yang@intel.com>
Tested-by: Matthew Rosato <mjrosato@linux.ibm.com>
Tested-by: Yu He <yu.he@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>