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Re: [Qemu-devel] [RFC][PATCH v5 00/21] virtagent: host/guest RPC communi
Re: [Qemu-devel] [RFC][PATCH v5 00/21] virtagent: host/guest RPC communication agent
Thu, 09 Dec 2010 14:45:17 -0600
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On 12/08/2010 04:10 AM, Stefan Hajnoczi wrote:
On Fri, Dec 3, 2010 at 6:03 PM, Michael Roth<address@hidden> wrote:
These patches apply to master, and can also be obtained from:
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
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.
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.
Really useful for trouble-shooting things like soft-lockups.
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
- [Qemu-devel] Re: [RFC][PATCH v5 19/21] virtagent: add virtagent guest daemon, (continued)
- [Qemu-devel] [RFC][PATCH v5 20/21] virtagent: integrate virtagent server/client via chardev, Michael Roth, 2010/12/03
- [Qemu-devel] [RFC][PATCH v5 15/21] virtagent: add agent_capabilities qmp/hmp commands, Michael Roth, 2010/12/03
- [Qemu-devel] [RFC][PATCH v5 16/21] virtagent: add client capabilities init function, Michael Roth, 2010/12/03
- [Qemu-devel] [RFC][PATCH v5 10/21] virtagent: add agent_viewdmesg qmp/hmp commands, Michael Roth, 2010/12/03
- [Qemu-devel] [RFC][PATCH v5 21/21] virtagent: various bits to build QEMU with virtagent, Michael Roth, 2010/12/03
- [Qemu-devel] Re: [RFC][PATCH v5 00/21] virtagent: host/guest RPC communication agent, Jes Sorensen, 2010/12/07
- Re: [Qemu-devel] [RFC][PATCH v5 00/21] virtagent: host/guest RPC communication agent, Stefan Hajnoczi, 2010/12/08