xen/sysfs: Use XENVER_guest_handle to query UUID

This hypercall has been present since Xen 3.1, and is the preferred
method for a domain to obtain its UUID. Fall back to the xenstore method
if using an older version of Xen (which returns -ENOSYS).
Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/sys-hypervisor.c

@@ -114,7 +114,7 @@ static void xen_sysfs_version_destroy(void)
 
 /* UUID */
 
-static ssize_t uuid_show(struct hyp_sysfs_attr *attr, char *buffer)
+static ssize_t uuid_show_fallback(struct hyp_sysfs_attr *attr, char *buffer)
 {
 	char *vm, *val;
 	int ret;
@@ -135,6 +135,17 @@ static ssize_t uuid_show(struct hyp_sysfs_attr *attr, char *buffer)
 	return ret;
 }
 
+static ssize_t uuid_show(struct hyp_sysfs_attr *attr, char *buffer)
+{
+	xen_domain_handle_t uuid;
+	int ret;
+	ret = HYPERVISOR_xen_version(XENVER_guest_handle, uuid);
+	if (ret)
+		return uuid_show_fallback(attr, buffer);
+	ret = sprintf(buffer, "%pU\n", uuid);
+	return ret;
+}
+
 HYPERVISOR_ATTR_RO(uuid);
 
 static int __init xen_sysfs_uuid_init(void)

include/xen/interface/version.h

@@ -60,4 +60,7 @@ struct xen_feature_info {
 /* arg == NULL; returns host memory page size. */
 #define XENVER_pagesize 7
 
+/* arg == xen_domain_handle_t. */
+#define XENVER_guest_handle 8
+
 #endif /* __XEN_PUBLIC_VERSION_H__ */