vfio/type1: Add extension to test DMA cache coherence of IOMMU

Now that the type1 IOMMU backend can support IOMMU_CACHE, we need to
be able to test whether coherency is currently enforced.  Add an
extension for this.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
index 460fdf2..cb9023d 100644
--- a/include/uapi/linux/vfio.h
+++ b/include/uapi/linux/vfio.h
@@ -24,6 +24,11 @@
 #define VFIO_TYPE1_IOMMU		1
 #define VFIO_SPAPR_TCE_IOMMU		2
 #define VFIO_TYPE1v2_IOMMU		3
+/*
+ * IOMMU enforces DMA cache coherence (ex. PCIe NoSnoop stripping).  This
+ * capability is subject to change as groups are added or removed.
+ */
+#define VFIO_DMA_CC_IOMMU		4
 
 /*
  * The IOCTL interface is designed for extensibility by embedding the