diff --git a/stand/efi/boot1/zfs_module.c b/stand/efi/boot1/zfs_module.c
--- a/stand/efi/boot1/zfs_module.c
+++ b/stand/efi/boot1/zfs_module.c
@@ -144,6 +144,7 @@
 	struct zfsmount zmount;
 	dnode_phys_t dn;
 	struct stat st;
+	uint64_t rootobj;
 	int err;
 	void *buf;
 
@@ -162,20 +163,49 @@
 		return (EFI_NOT_FOUND);
 	}
 
-	if ((err = zfs_mount_impl(spa, 0, &zmount)) != 0) {
-		DPRINTF("Failed to mount pool '%s' (%d)\n", spa->spa_name, err);
-		return (EFI_NOT_FOUND);
+	if (zfs_get_bootonce_spa(spa, OS_BOOTONCE, zfs_bootonce,
+	    sizeof(zfs_bootonce)) == 0) {
+		/*
+		 * If bootonce attribute is present, use it as root dataset.
+		 * Any attempt to use it should clear the 'once' flag.  Prior
+		 * to now, we'd not be able to clear it anyway.  We don't care
+		 * if we can't find the files to boot, or if there's a problem
+		 * with it: we've tried to use it once we're able to mount the
+		 * ZFS dataset.
+		 *
+		 * Note: the attribute is prefixed with "zfs:" and suffixed
+		 * with ":".
+		 */
+		char *dname, *end;
+
+		if (zfs_bootonce[0] != 'z' || zfs_bootonce[1] != 'f' ||
+		    zfs_bootonce[2] != 's' || zfs_bootonce[3] != ':' ||
+		    (dname = strchr(&zfs_bootonce[4], '/')) == NULL ||
+		    (end = strrchr(&zfs_bootonce[4], ':')) == NULL) {
+			printf("INVALID zfs bootonce: %s\n", zfs_bootonce);
+			*zfs_bootonce = '\0';
+			rootobj = 0;
+		} else {
+			dname += 1;
+			*end = '\0';
+			if (zfs_lookup_dataset(spa, dname, &rootobj) != 0) {
+				printf("zfs bootonce dataset %s NOT FOUND\n",
+				    dname);
+				*zfs_bootonce = '\0';
+				rootobj = 0;
+			} else
+				printf("zfs bootonce: %s\n", zfs_bootonce);
+			*end = ':';
+		}
+	} else {
+		*zfs_bootonce = '\0';
+		rootobj = 0;
 	}
 
-	/*
-	 * OK. We've found a filesystem. Any attempt to use it should clear the
-	 * 'once' flag. Prior to now, we'd not be able to clear it anyway. We
-	 * don't care if we can't find the files to boot, or if there's a
-	 * problem with it: we've tried to use it once we're able to mount the
-	 * ZFS dataset.
-	 */
-	*zfs_bootonce = '\0';
-	zfs_get_bootonce_spa(spa, OS_BOOTONCE, zfs_bootonce, sizeof(zfs_bootonce));
+	if ((err = zfs_mount_impl(spa, rootobj, &zmount)) != 0) {
+		printf("Failed to mount pool '%s' (%d)\n", spa->spa_name, err);
+		return (EFI_NOT_FOUND);
+	}
 
 	if ((err = zfs_lookup(&zmount, filepath, &dn)) != 0) {
 		if (err == ENOENT) {