kvm/vfio: Support for DMA coherent IOMMUs

VFIO now has support for using the IOMMU_CACHE flag and a mechanism
for an external user to test the current operating mode of the IOMMU.
Add support for this to the kvm-vfio pseudo device so that we only
register noncoherent DMA when necessary.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Cc: Gleb Natapov <gleb@kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/vfio.c

@@ -59,6 +59,22 @@ static void kvm_vfio_group_put_external_user(struct vfio_group *vfio_group)
 	symbol_put(vfio_group_put_external_user);
 }
 
+static bool kvm_vfio_group_is_coherent(struct vfio_group *vfio_group)
+{
+	long (*fn)(struct vfio_group *, unsigned long);
+	long ret;
+
+	fn = symbol_get(vfio_external_check_extension);
+	if (!fn)
+		return false;
+
+	ret = fn(vfio_group, VFIO_DMA_CC_IOMMU);
+
+	symbol_put(vfio_external_check_extension);
+
+	return ret > 0;
+}
+
 /*
  * Groups can use the same or different IOMMU domains.  If the same then
  * adding a new group may change the coherency of groups we've previously
@@ -75,13 +91,10 @@ static void kvm_vfio_update_coherency(struct kvm_device *dev)
 	mutex_lock(&kv->lock);
 
 	list_for_each_entry(kvg, &kv->group_list, node) {
-		/*
-		 * TODO: We need an interface to check the coherency of
-		 * the IOMMU domain this group is using.  For now, assume
-		 * it's always noncoherent.
-		 */
-		noncoherent = true;
-		break;
+		if (!kvm_vfio_group_is_coherent(kvg->vfio_group)) {
+			noncoherent = true;
+			break;
+		}
 	}
 
 	if (noncoherent != kv->noncoherent) {