Re: [RFC] Unify KVM kernel-space and user-space code into a single project

Daniel P. Berrange
2010-03-22T13:05:13Z

On Mon, Mar 22, 2010 at 01:54:40PM +0100, Ingo Molnar wrote:
>
> * Daniel P. Berrange <berrange@redhat.com> wrote:
> >
> > FYI, for offline guests, you can use libguestfs[1] to access & change files
> > inside the guest, and read-only access to running guests files. It provides
> > access via a interactive shell, APIs in all major languages, and also has a
> > FUSE mdule to expose it directly in the host VFS. It could probably be made
> > to work read-write for running guests too if its agent were installed inside
> > the guest & leverage the new Virtio-Serial channel for comms (avoiding any
> > network setup requirements).
> >
> > Regards,
> > Daniel
> >
> > [1] http://libguestfs.org/
>
> Yes, this is the kind of functionality i'm suggesting.
>
> I'd suggest a different implementation for live guests: to drive this from
> within the live guest side of KVM, i.e. basically a paravirt driver for
> guestfs. You'd pass file API guests to the guest directly, via the KVM ioctl
> or so - and get responses from the guest.
>
> That will give true read-write access and completely coherent (and still
> transparent) VFS integration, with no host-side knowledge needed for the
> guest's low level (raw) filesystem structure. That's a big advantage.
>
> Yes, it needs an 'aware' guest kernel - but that is a one-off transition
> overhead whose cost is zero in the long run. (i.e. all KVM kernels beyond a
> given version would have this ability - otherwise it's guest side distribution
> transparent)
>
> Even 'offline' read-only access could be implemented by booting a minimal
> kernel via qemu -kernel and using a 'ro' boot option. That way you could
> eliminate all lowlevel filesystem knowledge from libguestfs. You could run
> ext4 or btrfs guest filesystems and FAT ones as well - with no restriction.
>
> This would allow 'offline' access to Windows images as well: a FAT or ntfs
> enabled mini-kernel could be booted in read-only mode.

This is close to the way libguestfs already works. It boots QEMU/KVM pointing
to a minimal stripped down appliance linux OS image, containing a small agent
it talks to over some form of vmchannel/serial/virtio-serial device. Thus the
kernel in the appliance it runs is the only thing that needs to know about the
filesystem/lvm/dm on-disk formats - libguestfs definitely does not want to be
duplicating this detailed knowledge of on disk format itself. It is doing
full read-write access to the guest filesystem in offline mode - one of the
major use cases is disaster recovery from a unbootable guest OS image.

Regards,
Daniel
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