Messages in this thread | | | Date | Mon, 14 Jun 2021 18:01:44 +0900 | From | Dominique Martinet <> | Subject | Re: [PATCH v3 0/3] 9p: add support for root file systems |
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Josh Triplett wrote on Mon, Jun 14, 2021 at 01:57:54AM -0700: > 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.
This doesn't work as is because of the way the code is written, if there's no block device associated with a root=x option right now it will lead to kernel panic.
I replied with folks in Cc but there's another thread on linux-fsdevel@ with a more generic approach that will build a list of filesystems which don't require such a block device (either hardcoded with virtiofs and 9p or based on FS_REQUIRES_DEV), thread started there but there's a second patch hidden an more discussion below: https://lore.kernel.org/linux-fsdevel/20210608153524.GB504497@redhat.com/
My preferred approach right now would be to go with their approach, and adjust the documentation Changbin Du wrote here, to have the best of both worlds.
-- Dominique
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