Bug#696292: For 9p root filesystems, kernel ignores root=$NAME option on command line, and uses /dev/root as mount tag

[Josh Triplett <josh@joshtriplett.org>]

Control: reassign -1 src:linux 3.6.9-1~experimental.1

On Wed, Dec 19, 2012 at 12:38:55AM +0000, Ben Hutchings wrote:
> Control: reassign -1 initramfs-tools 0.109
> 
> On Tue, 2012-12-18 at 16:20 -0800, Josh Triplett wrote:
> > Package: src:linux
> > Version: 3.6.9-1~experimental.1
> > Severity: normal
> > 
> > The virtio 9p filesystem uses a "mount tag" to distinguish between
> > multiple filesystems provided to the guest.  The host can provide these
> > via the mount_tag=$NAME parameter to kvm's -virtfs option, and the guest
> > supplies the mount tag as the device name to mount.  This works fine for
> > non-root filesystems.  However, when attempting to use a virtio 9p
> > filesystem as the root filesystem (with rootfstype=9p), the kernel
> > ignores the "root=$NAME" option on the kernel command line, and always
> > uses "/dev/root" as the mount tag.  Making matters worse, kvm itself
> > doesn't currently accept "/dev/root" as a mount tag, or anything else
> > with a '/' in it (separate bug filed for that).  Hacking kvm to accept
> > /dev/root allows this to work, but the kernel really should use the name
> > passed to the root= option as the mount tag.
> [...]
> 
> When using an initramfs, implementation of the 'root' kernel parameter
> is the responsibility of the initramfs!

I'm not using an initramfs.  The kernel supports 9p root filesystems
without one, apart from using "/dev/root" as the mount tag instead of
the value from the kernel command line.

- Josh Triplett