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

[Luiz Capitulino]

On Thu, 05 Jan 2012 09:10:50 -0600
Michael Roth <address@hidden> wrote:

> On 01/05/2012 08:42 AM, Luiz Capitulino wrote:
> > On Thu, 5 Jan 2012 12:59:27 +0000
> > "Daniel P. Berrange"<address@hidden>  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.
> >
> > Yes, I agree. As a side note: if we add error reporting it will only work
> > on 1.1 and later.  Ie, the problem you describe above will still happen
> > with 1.0.
> >
> > But what you're suggesting seems to be the right thing to do. Do you agree
> > Michael?
> 
> Agree, but unless we add an RPC that QEMU uses to advertise 
> capabilities, I'm really not sure it's possible to detect whether or not 
> the host will support it.

You mean an RPC to advertise if 'sleep' is supported? I think this is best done
by making guest-suspend return an error as suggested by Daniel, otherwise a
client that doesn't query for capabilities might run in trouble.

There's an important detail though: we need to make qemu not advertise S3 for
this to work. However, we might be able to fix S3 for 1.1 (and bugs, like the
S4 ones, can't be detected, limiting the scope of the 'unsupported' error).

So, we could merge all modes and commit to get S3 fixed for 1.1 :)

> And if we can't detect that reliably, we're 
> better off leaving it out for now, because sleeping guests is not 
> obscure functionality, and accidentally nuking guests when a user sleeps 
> them (presumably because they want to retain their working state) is 
> much worse than telling a user to upgrade their agent, or not supported 
> or whatever.
> 
> >
> >> 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.
> >
>