Re: [PATCH v3 0/3] 9p: add support for root file systems

[Josh Triplett]

On Mon, Jun 07, 2021 at 07:09:19AM +0800, Changbin Du wrote:
> Just like cifs and nfs, this short series enables rootfs support for 9p.
> Bellow is an example which mounts v9fs with tag 'r' as rootfs in qemu
> guest via virtio transport.
> 
>   $ qemu-system-x86_64 -enable-kvm -cpu host -m 1024 \
>         -virtfs local,path=$rootfs_dir,mount_tag=r,security_model=passthrough,id=r \
>         -kernel /path/to/linux/arch/x86/boot/bzImage -nographic \
>         -append "root=/dev/v9fs v9fsroot=r,trans=virtio rw console=ttyS0 3"

Rather than inventing a pseudo-device /dev/v9fs for this, would it
potentially work to use the existing rootfstype and rootflags options
for this? rootfstype already determines what filesystem should be used
to mount the root, and rootflags already provides options for that
filesystem.

For instance, for the above example:
rootfstype=9p root=r rootflags=trans=virtio

That would require a bit of fiddling to make rootfstype=9p allow a root
that's just the mount_tag. If that isn't an option, then even with
root=/dev/v9fs I think it still makes sense to use the existing
rootflags for "trans=virtio" rather than creating a new "v9fsroot"
option for that.

- Josh Triplett