Re: [Qemu-devel] [PATCH v4 0/2]: qemu-ga: Add the guest-suspend command

[Daniel P. Berrange]

On Thu, Jan 05, 2012 at 09:04:57AM -0600, Michael Roth wrote:
> On 01/05/2012 06:59 AM, Daniel P. Berrange wrote:
> >On Thu, Jan 05, 2012 at 10:37:14AM -0200, Luiz Capitulino wrote:
> >>On Thu, 5 Jan 2012 10:16:30 +0000
> >>"Daniel P. Berrange"<address@hidden>  wrote:
> >>
> >>>On Wed, Jan 04, 2012 at 05:45:11PM -0200, Luiz Capitulino wrote:
> >>>>This version drops modes 'sleep' and 'hybrid' because they don't work
> >>>>properly due to issues in qemu. Only the 'hibernate' mode is supported
> >>>>for now.
> >>>
> >>>IMHO this is short-sighted. When the bugs QEMU in are fixed so that
> >>>these modes work, you have needlessly put users in the situation where
> >>>they have to now upgrade the guest agent everywhere to take advantage
> >>>of the bugfix.
> >>
> >>That was my thinking until v4. But after discussing with Michael the issues
> >>we have with S3 I concluded that it doesn't make sense to offer an API to
> >>something that doesn't work, this will just generate bug reports. Also,
> >>updating to get new features is normal and expected.
> >
> >This is assuming that users will always upgrade their VMs&  hosts in
> >lock step, which I rather doubt they will in practice. eg imagine a
> >deployment might have a mixture of hosts, running QEMU 1.1 (broken S3)
> >and QEMU 1.2 (working S3). If they build VM disk images they will likely
> >use the QEMU GA from 1.2 for all their builds, even if many of them
> >will only run on QEMU 1.1 hosts. So you'll end up having 'sleep' and
> >'hybrid' commands available in the guest agent, even though the host
> >QEMU doesn't work properly.
> >
> >So you *will* ultimately need to make sure that QEMU GA from 1.2, has
> >sensible behaviour when run on a QEMU 1.1 host.  If you don't address
> >this during 1.1, you may well find yourself in an un-winnable situation
> >for 1.2, where it is impossible to provide good behaviour on old hosts.
> >
> >So IMHO we are better off in the long run, if we include all commands
> >right now, even though some don't work yet, and work to ensure we have
> >good error reporting behaviour for those that don't work.
> >
> >As an example, if S3 is broken in current QEMU, then we should not be
> >advertizing S3 to the guest OS. This would allow 'pm-is-supported --suspend'
> >to return false, at which point the guest agent can send back a nice error
> >message 'Suspend is not supported on this host', instead of just having the
> >guest try to suspend&  hang or worse.
> 
> This still requires we're lockstep with host QEMU (ideally that
> would be the case via push-deployment of the agent, exactly because
> of issues like this. Or at least, it'd make the upgrade process
> painless). And outside of that, I really cannot think of any nice
> way to check, from the agent, that a host has required functionality
> for {this,an} RPC. Not unless we forced a bi-directional
> capabilities negotiation sequence, and I don't like the idea of
> injecting this kind of data into a guest. libvirt could maybe filter
> the modes based on QEMU version, but that's not the only consumer of
> the agent.

Err, the scenario I just described does not require lockstep
upgrade. Newer QEMU GA agent should be able to run on historical
QEMU hosts just fine. I'm also not trying to suggest we need a
general bi-directional capabilities negotiation here either.
The key is that in this particular case, QEMU should only
expose S3 to the guest if it is actually capable of working.
Then, the pm-is-supported  command will 'just work'. No
host<->guest agent negoiation is required.

Daniel
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