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

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Date:	Mon, 22 Mar 2010 13:54:40 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Daniel P. Berrange" <berrange@...hat.com>
Cc:	Pekka Enberg <penberg@...helsinki.fi>, Avi Kivity <avi@...hat.com>,
	Antoine Martin <antoine@...afix.co.uk>,
	Olivier Galibert <galibert@...ox.com>,
	Anthony Liguori <anthony@...emonkey.ws>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Sheng Yang <sheng@...ux.intel.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	oerg Roedel <joro@...tes.org>,
	Jes Sorensen <Jes.Sorensen@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>, ziteng.huang@...el.com,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Fr?d?ric Weisbecker <fweisbec@...il.com>
Subject: Re: [RFC] Unify KVM kernel-space and user-space code into a single
 project


* Daniel P. Berrange <berrange@...hat.com> wrote:

> On Mon, Mar 22, 2010 at 02:31:49PM +0200, Pekka Enberg wrote:
> > On Mon, Mar 22, 2010 at 1:48 PM, Ingo Molnar <mingo@...e.hu> wrote:
> > >> What about line number information? ?And the source? ?Into the kernel with
> > >> them as well?
> > >
> > > Sigh. Please read the _very first_ suggestion i made, which solves all that. I
> > > rarely go into discussions without suggesting technical solutions - i'm not
> > > interested in flaming, i'm interested in real solutions.
> > >
> > > Here it is, repeated for the Nth time:
> > >
> > > Allow a guest to (optionally) integrate its VFS namespace with the host side
> > > as well. An example scheme would be:
> > >
> > > ? /guests/Fedora-G1/
> > > ? /guests/Fedora-G1/proc/
> > > ? /guests/Fedora-G1/usr/
> > > ? /guests/Fedora-G1/.../
> > > ? /guests/OpenSuse-G2/
> > > ? /guests/OpenSuse-G2/proc/
> > > ? /guests/OpenSuse-G2/usr/
> > > ? /guests/OpenSuse-G2/.../
> > >
> > > ?( This feature would be configurable and would be default-off, to maintain
> > > ? ?the current status quo. )
> > 
> > Heh, funny. That would also solve my number one gripe with virtualization 
> > these days: how to get files in and out of guests without having to 
> > install extra packages on the guest side and fiddling with mount points on 
> > every single guest image I want to play with.
> 
> 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.

Thanks,

	Ingo
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