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

[Daniel P. Berrange]

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.

Daniel
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