loader: extend root fs auto-discovery

This extends the (fallback) root file system auto-discovery mechanism to
include virtio-fs, so now the search order becomes rofs -> virtio-fs ->
zfs.

While at it, we also ensure that when even zfs fails, we mount the fstab
entries before continuing with ramfs as the root file system.

Signed-off-by: Fotis Xenakis <[email protected]>
Message-Id: <VI1PR03MB3773F53D7D312B17CEDFF3BBA67F9@VI1PR03MB3773.eurprd03.prod.outlook.com>

loader.cc

@@ -444,23 +444,29 @@ void* do_main_thread(void *_main_args)
 
             boot_time.event("Virtio-fs mounted");
         } else {
-            // Fallback to original behavior for compatibility: try rofs -> zfs
+            // Auto-discovery: try rofs -> virtio-fs -> ZFS
             if (mount_rofs_rootfs(opt_pivot) == 0) {
                 if (opt_disable_rofs_cache) {
                     debug("Disabling ROFS memory cache.\n");
                     rofs_disable_cache();
                 }
                 boot_time.event("ROFS mounted");
+            } else if (mount_virtiofs_rootfs(opt_pivot) == 0) {
+                boot_time.event("Virtio-fs mounted");
             } else {
                 zfsdev::zfsdev_init();
                 auto error = mount_zfs_rootfs(opt_pivot, opt_extra_zfs_pools);
                 if (error) {
                     debug("Could not mount zfs root filesystem (while "
                           "auto-discovering).\n");
+                    // Continue with ramfs (already mounted)
+                    // TODO: Avoid the hack of using pivot_rootfs() just for
+                    // mounting the fstab entries.
+                    pivot_rootfs("/");
+                } else {
+                    bsd_shrinker_init();
+                    boot_time.event("ZFS mounted");
                 }
-
-                bsd_shrinker_init();
-                boot_time.event("ZFS mounted");
             }
         }
     }