[Virtio-fs] [PATCH] init/do_mounts.c: add virtiofs root fs support

[Richard Weinberger]

On Fri, Sep 6, 2019 at 1:15 PM Stefan Hajnoczi <stefanha at redhat.com> wrote:
>
> 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.
>
> Signed-off-by: Stefan Hajnoczi <stefanha at redhat.com>
> ---
> This patch is based on linux-next (next-20190904) but should apply
> cleanly to other virtiofs trees.
>
>  init/do_mounts.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/init/do_mounts.c b/init/do_mounts.c
> index 9634ecf3743d..030be2f1999a 100644
> --- a/init/do_mounts.c
> +++ b/init/do_mounts.c
> @@ -554,6 +554,16 @@ void __init mount_root(void)
>                         change_floppy("root floppy");
>         }
>  #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

I think you don't need this, you can abuse a hack for mtd/ubi in
prepare_namespace().
At least for 9p it works well:
qemu-system-x86_64 -m 4G -M pc,accel=kvm -nographic -kernel
arch/x86/boot/bzImage -append "rootfstype=9p
rootflags=trans=virtio,version=9p2000.L root=mtdfake console=ttyS0 ro
init=/bin/sh" -virtfs
local,id=rootfs,path=/,security_model=passthrough,mount_tag=mtdfake

If this works too for virtiofs I suggest to cleanup the hack and
generalize it. B-)

-- 
Thanks,
//richard