[RESEND v2 PATCH] init/do_mounts.c: add virtiofs root fs support

[David Heidelberg <david@ixit.cz>]

From: Stefan Hajnoczi <stefanha@redhat.com>

Make it possible to boot directly from a virtiofs file system with tag
'myfs' using the following kernel parameters:

  rootfstype=virtiofs root=myfs rw

Booting directly from virtiofs makes it possible to use a directory on
the host as the root file system.  This is convenient for testing and
situations where manipulating disk image files is cumbersome.

Reviewed-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: David Heidelberg <david@ixit.cz>
---
v2: added Reviewed-by and CCed everyone interested.

We have used this option in Mesa3D CI for testing crosvm for
more than one years and it's proven to work reliably.

We are working on effort to removing custom patches to be able to do 
automated apply and test of patches from any tree.                              

https://gitlab.freedesktop.org/mesa/mesa/-/blob/main/.gitlab-ci/crosvm-runner.sh#L85
 init/do_mounts.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/init/do_mounts.c b/init/do_mounts.c
index 811e94daf0a8..11c11abe23d7 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -578,6 +578,16 @@ void __init mount_root(void)
 			printk(KERN_ERR "VFS: Unable to mount root fs via SMB.\n");
 		return;
 	}
+#endif
+#ifdef CONFIG_VIRTIO_FS
+	if (root_fs_names && !strcmp(root_fs_names, "virtiofs")) {
+		if (!do_mount_root(root_device_name, "virtiofs",
+				   root_mountflags, root_mount_data))
+			return;
+
+		panic("VFS: Unable to mount root fs \"%s\" from virtiofs",
+		      root_device_name);
+	}
 #endif
 	if (ROOT_DEV == 0 && root_device_name && root_fs_names) {
 		if (mount_nodev_root() == 0)
-- 
2.39.1