1 files changed, 62 insertions, 0 deletions

diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
index 842f4507883e..d3cb34224369 100644
--- a/drivers/vfio/vfio.c
+++ b/drivers/vfio/vfio.c
@@ -1353,6 +1353,68 @@ static const struct file_operations vfio_device_fops = {
 };
 
 /**
+ * External user API, exported by symbols to be linked dynamically.
+ *
+ * The protocol includes:
+ *  1. do normal VFIO init operation:
+ *      - opening a new container;
+ *      - attaching group(s) to it;
+ *      - setting an IOMMU driver for a container.
+ * When IOMMU is set for a container, all groups in it are
+ * considered ready to use by an external user.
+ *
+ * 2. User space passes a group fd to an external user.
+ * The external user calls vfio_group_get_external_user()
+ * to verify that:
+ *      - the group is initialized;
+ *      - IOMMU is set for it.
+ * If both checks passed, vfio_group_get_external_user()
+ * increments the container user counter to prevent
+ * the VFIO group from disposal before KVM exits.
+ *
+ * 3. The external user calls vfio_external_user_iommu_id()
+ * to know an IOMMU ID.
+ *
+ * 4. When the external KVM finishes, it calls
+ * vfio_group_put_external_user() to release the VFIO group.
+ * This call decrements the container user counter.
+ */
+struct vfio_group *vfio_group_get_external_user(struct file *filep)
+{
+        struct vfio_group *group = filep->private_data;
+
+        if (filep->f_op != &vfio_group_fops)
+                return ERR_PTR(-EINVAL);
+
+        if (!atomic_inc_not_zero(&group->container_users))
+                return ERR_PTR(-EINVAL);
+
+        if (!group->container->iommu_driver ||
+                        !vfio_group_viable(group)) {
+                atomic_dec(&group->container_users);
+                return ERR_PTR(-EINVAL);
+        }
+
+        vfio_group_get(group);
+
+        return group;
+}
+EXPORT_SYMBOL_GPL(vfio_group_get_external_user);
+
+void vfio_group_put_external_user(struct vfio_group *group)
+{
+        vfio_group_put(group);
+        vfio_group_try_dissolve_container(group);
+}
+EXPORT_SYMBOL_GPL(vfio_group_put_external_user);
+
+int vfio_external_user_iommu_id(struct vfio_group *group)
+{
+        return iommu_group_id(group->iommu_group);
+}
+EXPORT_SYMBOL_GPL(vfio_external_user_iommu_id);
+
+/**
  * Module/class support
  */
 static char *vfio_devnode(struct device *dev, umode_t *mode)