[Qemu-devel] Re: [RFC][PATCH 00/10] virtagent: host/guest RPC communication agent

[Andrew Beekhof <abeekhof@redhat.com>]

On 10/25/2010 07:06 PM, Michael Roth wrote:
> On 10/25/2010 05:30 AM, Andrew Beekhof wrote:
>> On 10/22/2010 08:45 PM, Michael Roth wrote:
>>> This set of patches is meant to be applied on top of the Virtproxy v1
>>> patchset.
>>>
>>> OVERVIEW:
>>>
>>> There are a wide range of use cases motivating the need for a guest
>>> agent of some sort to extend the functionality/usability/control
>>> offered by QEMU.
>>> Some examples include graceful guest shutdown/reboot and notifications
>>> thereof, copy/paste syncing between host/guest, guest statistics
>>> gathering, file access, etc.
>>>
>>> Ideally these would all be served by a single, easilly extensible
>>> agent that can be deployed in a wide range of guests.
>>> Virtagent is an XMLRPC server integrated into the Virtproxy guest
>>> daemon and aimed at providing this type of functionality.
>>>
>>> This code is very rough, and I'll to document most of the
>>> bugs/shortcomings we're aware of in this version of the patchset.
>>> The main goal of this RFC to get feedback on the types of core
>>> functionality we would need in an agent of this sort, as well as
>>> feedback on the general approach/architecture implemented here.
>>> Any feedback is greatly appreciated however.
>>>
>>> To start off this discussion, there have been some recent posts about
>>> how much an agent of this sort overlaps with the goals of the Matahari
>>> project (https://fedorahosted.org/matahari/).
>>> While both of these approaches are at least *feasible*, our use cases
>>> require the ability to deploy to guests which may not support
>>> virtio-serial, which currently rules Matahari out.
>>> This support could be added however: the virtproxy layer used by this
>>> agent actually lends itself to extending such support to other
>>> agents/services, or a more direct approach could be taken in adding
>>> support for isa-serial.
>>>
>>> The question that remains however is one of scope.
>>> This agent is intended purely as a means to extend qemu's abilities to
>>> perform hypervisor-specific work,
>>
>> "shutdown/reboot", "statistics", "file gathering"... none of those sound
>> very "hypervisor-specific" to me ;-)
>>
>
> QEMU-specific is probably a better word, in the sense that the API (and
> the code dependencies) terminates at the QEMU level.

The concepts though apply to all flavours of virtualization and even 
bare-metal machines.

Implementation details aside, they're the first functions any 
agent/collection of agents is going to implement - which is why I'm 
having a hard time with the *-specific labels.

> Though I think one
> can make the argument that shutdown/reboot are hypervisor-related,
> especially given that there are situations where we currently cannot
> induce a guest reboot from qemu. Also, file access is actually horrible
> use case for RPC. It does however provide a pretty powerful primitive
> for exposing data/statistics made available in /proc or /sys, which do
> extend a hypervisor's ability to make better decisions about things like
> memory ballooning.
>
> But those are just some examples of the functionality that could be
> implemented by the agent, and the RPCs that make sense to implement are
> TBD based on input here and elsewhere.
>
>>> whereas Matahari aims to extend general system management capabilities
>>> to guests (please correct me if I'm oversimplifying).
>>
>> As I replied elsewhere, Matahari is both an architecture and a
>> collection of independent but commonly useful agents.
>>
>> So while there will be a bunch of other agents doing a bunch of things
>> you don't care about, you don't have to care that they exist either :-)
>>
>> A hypothetical QEMU agent would be a independent entity, with both the
>> daemon and source code completely isolated from any other agents.
>>
>> It doesn't even need to live in the Matahari project.
>>
>  >> Virtagent cannot meet Matahari's goals, whereas Matahari technically
>  >> can meet Virtagent's.
>  >> My contention however is that the qemu-specific scope/API and shared
>  >> code base with a more closely integrated agent will provide a more
>  >> expedient route to functional improvements to qemu,
>  >
>  > See above. Would leveraging the Matahari architecture but keeping the
>  > agent in the QEMU project address this concern?
>  >
>
> To some extent. But what is gained here though except the bus?

I guess I was thinking that if you have the bus, why not make use of the 
existing standardised agents and focus the QEMU one on the parts that 
weren't already provided.

> If we
> aren't using the functions provided by the common agents we still need
> to re-implement them,  and if we do use them then we are dependent on
> their implementations as well as their deployment overhead. And even if
> we do choose to ignore the existing agents, we inherit the guest and
> host-side dependencies of Matahari nonetheless. Maybe these dependencies
> are fairly minimal, but it makes it that much more difficult to support
> other/older guests.

Matahari is purely guest side, the Host is currently a standard qpid 
server.  For usage on a single single host its possible we may drop that 
as a hard requirement and support dbus as the host-side aggregator.

On the guest side, each agent may well end up being its own subpackage.
So the dependency list would depend on what functionality the user 
wanted (plus qpid of course).

>
> Beyond deployment, there's also just the basic end-user experience we
> need to consider. In the context of general systems management it makes
> total sense that you'd install Matahari and then whatever agents you
> needed in your guests/hosts. In the context of qemu it doesn't. VMWare
> has a vmware package that interacts with vmware, virtualbox has a
> virtualbox utilities package that interacts with virtualbox.

That's kinda the point, everyone is reinventing the wheel here.
And not just virt projects, off the top of my head there are a couple 
for clustering; one for SAP, Puppet, MRG; the FMCI team is working on a 
bunch...

We're working with all those groups (and others) to try and get some 
consolidation.  Mostly the reaction so far has been positive but if you 
want to do your own thing, I can live with that.

What would be nice though is if the guest-side functions that implement 
the RPC calls lived in a library.  That way, like we do with netcf and 
libvirt, we could write glue for plugging the QEMU-specific ones into 
Matahari.