Re: [Qemu-devel] converging around a single guest agent

[Adam Litke]

On Thu, Nov 17, 2011 at 03:46:37AM -0500, Ayal Baron wrote:
> 
> 
> ----- Original Message -----
> > I have been following this thread pretty closely and the one sentence
> > summary of the current argument is: ovirt-guest-agent is already featureful
> > and tested, so let's drop qemu-ga and have everyone adopt ovirt-guest-agent.
> 
> What we're suggesting is let's drop *one* of the two agents (obviously it
> would be easier for us to drop qemu-ga, but we'd rather reach consensus and
> unite behind one agent regardless of which agent it is).
> 
> >  Unfortunately, this track strays completely away from the stated goal of
> >  convergence.  I have at least two examples of why the greater KVM community
> >  can never adopt ovirt-guest-agent as-is.  To address this, I would like to
> >  counter with an example on how qemu-ga can enable the deployment of
> >  ovirt-guest-agent features and satisfy the needs of the whole community at
> >  the same time.
> > 
> > 1) Scope:  The ovirt-guest-agent contains functionality that is incredibly
> > useful within the context of oVirt.  Single Sign-on is very handy but KVM
> > users outside the scope of oVirt will not want this extra complexity in
> > their agent.  For simplicity they will probably just write something small
> > that does what they need (and we have failed to provide a ubiquitous KVM
> > agent).
> 
> I totally agree, but that could easily be resolved using the plugin
> architecture suggested before.
> 
> > 
> > 1) Deployment complexity: The more complex the guest agent is, the more
> > often it will need to be updated (bug/security fixes, distro compatibility,
> > new features).  Rolling out guest agent updates does not scale well in large
> > environments (especially when the guest and host administrators are not the
> > same person).
> 
> Using plugins, you just deploy the ones you need, keeping the attack surface /
> #bugs / need to update lower

In order for any KVM guest agent to become ubiquitous, I think the code _must_ 
live
in the qemu repository.  This includes the base infrastructure and a core set of
plugins to provide the current set of qemu-ga APIs.  This way, both endpoints
(host/guest) can evolve together.  How easy would it be to extract this basic
infrastructure from the ovirt-guest-agent?  Is the qemu project opposed to a
Python agent?

> > For these reasons (and many others), I support having an agent with very
> > basic primitives that can be orchestrated by the host to provide needed
> > functionality.  This agent would present a low-level, stable, extensible API
> > that everyone can use.  Today qemu-ga supports the following verbs: sync
> > ping info shutdown file-open file-close file-read file-write file-seek
> > file-flush fsfreeze-status fsfreeze-freeze fsfreeze-thaw.  If we add a
> > generic execute mechanism, then the agent can provide everything needed by
> > oVirt to deploy SSO.
> > 
> > Let's assume that we have already agreed on some sort of security policy for
> > the write-file and exec primitives.  Consensus is possible on this issue but
> > I don't want to get bogged down with that here.
> > 
> > With the above primitives, SSO could be deployed automatically to a guest
> > with the following sequence of commands:
> > 
> > file-open "<exec-dir>/sso-package.bin" "w" file-write <fh> <buf> file-close
> > <fh> file-open "<exec-dir>/sso-package.bin" "x" file-exec <fh> <args>
> > file-close <fh>
> 
> The guest can run on any number of hosts.  currently, the guest tools contain
> all the relevant logic installed (specifically for the guest os version).
> What you're suggesting here is that we keep all the relevant guest-agent
> variants code on the host, automatically detect the guest os version and
> inject the correct file (e.g. SSO on winXP and on win2k8 is totally
> different).  In addition, there might be things requiring boot for example. So
> to solve that we would instead need to install a set of tools on the guest
> like we do the guest agent today (it would be a separate package because it's
> management specific).  And then we would tell the guest-agent to run tools
> from that set?  Sounds overly complex to me.

We already have that packaging complexity today.  You must already maintain the
various Windows packages somewhere.  You'd just be pushing them from the host
instead.  Could you provide examples of the things required for boot?  If you
are talking virtio drivers, I think this is a separate problem.  I would argue
that vdsm should have a hardware "safe-mode" when the guest tools are not
installed.  This would be a set of hardware exposed that is known to work with
all guests.  Then, when the guest tools are installed, the hardware can be
"upgraded" since we will know the guest can support paravirt hw.

> > At this point, the package is installed.  It can contain whatever existing
> > logic exists in the ovirt-guest-agent today.  To perform a user login, we'll
> > assume that sso-package.bin contains an executable 'sso/do-user-sso':
> > 
> > file-open "<exec-dir>/sso/do-user-sso" "x" exec <fh> <args> file-close <fh>
> > 
> > At this point the user would be logged in as before.
> > 
> > Obviously, this type of approach could be made easier by providing a well
> > designed exec API that returns command exit codes and (optionally) command
> > output.  We could also formalize the install of additional components into
> > some sort of plugin interface.  These are all relatively easy problems to
> > solve.
> > 
> > If we go in this direction, we would have a simple, general-purpose agent
> > with low-level primitives that everyone can use.  We would also be able to
> > easily extend the agent based on the needs of individual deployments (not
> > the least of which is an oVirt environment).  If certain plugins become
> > popular enough, they can always be promoted to first-order API calls in
> > future versions of the API.
> > 
> > What are your thoughts on this approach?
> > 
> > -- Adam Litke <address@hidden> IBM Linux Technology Center
> > 
> > _______________________________________________ Arch mailing list
> > address@hidden http://lists.ovirt.org/mailman/listinfo/arch
> > 
> 

-- 
Adam Litke <address@hidden>
IBM Linux Technology Center