Re: [Qemu-devel] [RFC][PATCH v5 00/21] virtagent: host/guest RPC communication agent

[Michael Roth <mdroth@linux.vnet.ibm.com>]

On 12/08/2010 04:10 AM, Stefan Hajnoczi wrote:
> On Fri, Dec 3, 2010 at 6:03 PM, Michael Roth<mdroth@linux.vnet.ibm.com>  wrote:
>> These patches apply to master, and can also be obtained from:
>> git://repo.or.cz/qemu/mdroth.git virtagent_v5
>
> Why XML-RPC and not QMP?  When I skim through the patch series it
> seems like much of the work being done is very similar to QMP.
>

It does, actually, more than I realized. In terms of data encapsulation 
I don't see why we couldn't use QMP/JSON for at least the current set of 
RPCs. XMLRPC does support a wider range of data types however, such as 
nestable arrays and structs, and set-precision numerical types like 32 
and 64-bit ints/floats, which may prove useful for future lower level 
interfaces.

QMP can be fairly trivially extended for some of these things, but JSON 
is a limiting factor in some cases. We can't maintain strong typing of 
data across the transport for instance, since JSON only allows for a 
generic number type, whereas with XMLRPC we can explicitly specify the 
machine representation. But whether this really matters is hard to say 
with the limited set of RPCs we have right now.

In terms of using QMP all the way through...that sounds really cool, but 
with the bi-directional client/server data being multiplexed over a 
single channel, and increased potential for weird states resulting from 
guest/guest agent restarts, there's a need for some additional 
synchronization that might end up being fairly intrusive/undesirable to 
work into QMP.

> What concrete use-cases are there?
> * Reboot support on x86.  A QMP command can invoke guest-initiated
> reboot via virtagent.
> * ?
>

* viewfile
The ability to do a quick peek at guest stats via, say, /proc, is a use 
case that seems to be fairly generally desirable. It's what essentially 
started all this work, actually. That's also why I think a 
simple/limited viewfile RPC for relatively small text files is warranted 
regardless of whatever approach we end up taking for handling 
large/binary file transfers.

* getdmesg
Really useful for trouble-shooting things like soft-lockups.

* ping
Heartbeat monitoring has also been a fairly re-occurring approach to 
identifying potential problems in our cloud, and it's not even something 
we're capable of accomplishing in a production environment due to having 
limited network access to the guests. Being able to do it without 
relying on custom/network-based daemons would be pretty useful.

* exec (planned)
Internally and externally I've seen interest in guest-initiated 
snapshots, but that would tie into exec or some other, higher-level, 
RPC, which isn't yet well-defined.

* copyfile (planned)
Nothing solid, but I think it's generally desirable. Quick access to 
coredumps and such would be useful. Lots of discussion on how to 
implement this and I think we have some good potential approaches to 
adding this soon.

* writefile (planned)
Nothing solid. But guest activation is probably a big potential use case 
for this one. Managing various guest kernel params is another. Another 
would be deploying custom scripts to run via exec.

> Will virtagent be extensible by host administrators or end-users?  For
> example, can I drop in a custom command to collect statistics and
> invoke it across VMs on my hosts?  Do I need to recompile QEMU and/or
> the virtagent daemon?

writefile + exec would probably be the best way to achieve this. I don't 
think there are any plans to make the supported set of RPCs 
pluggable/extendable.

>
> Stefan