Re: [Qemu-devel] [PATCH 0/6] qapi: Convert migrate

[Luiz Capitulino]

On Wed, 15 Feb 2012 18:39:31 +0100
Jan Kiszka <address@hidden> wrote:

> On 2012-02-15 18:23, Luiz Capitulino wrote:
> > On Wed, 15 Feb 2012 14:34:52 +0100
> > Jan Kiszka <address@hidden> wrote:
> > 
> >> On 2012-02-15 13:49, Luiz Capitulino wrote:
> >>> On Wed, 15 Feb 2012 09:59:07 +0100
> >>> Jan Kiszka <address@hidden> wrote:
> >>>
> >>>> On 2012-02-10 20:31, Luiz Capitulino wrote:
> >>>>> This is a rebase of Anthony's conversion, from his glib branch; and 
> >>>>> this is
> >>>>> also the beginning of the conversion of complex commands to the qapi.
> >>>>>
> >>>>> There are two important changes that should be observed:
> >>>>>
> >>>>>  1. patch 5/6 purges the 'mon' object from migration code. One of the
> >>>>>     consequences is that we lose the ability to print progress status to
> >>>>>     the HMP user (esp. in block migration)
> >>>>
> >>>> This smells extremely fishy. You have some common "monitor" context in
> >>>> both cases, means something that decides where suspend/resume takes
> >>>> effect or where to pick up file descriptors from. If the exiting Monitor
> >>>> object is not generic enough, introduce some super-class and use that in
> >>>> common services. Or make sure that a variant of Monitor is also valid
> >>>> over QMP. But don't remove the dependency from the API, while
> >>>> reintroducing it via the backdoor of cur_mon.
> >>>
> >>> What we really want to do here is to untangle HMP and QMP. Unfortunately,
> >>> the migrate command is one of those commands where the two are deeply
> >>> tangled and the split won't be perfect.
> >>>
> >>> However, the two cases you mention above are solvable:
> >>>
> >>>  1. suspend/resume: this is *really* a HMP feature and shouldn't be in any
> >>>     QMP code path. This is correctly addressed in this series by moving it
> >>>     to hmp_migrate()
> >>
> >> Almost correctly. ;)
> > 
> > Well, it was moved to the right place :)
> 
> (see the other thread)

Yeah, I saw it and will fix the problems you've pointed out.

> >>>  2. file descriptor passing: the new QMP server will support sessions and
> >>>     we'll move statefull commands (like getfd) to it. When we do it, we'll
> >>>     introduce a new API to get fds that won't depend on the monitor. 
> >>> However,
> >>>     this requires all commands to be converted to the qapi first. 
> >>> Meanwhile
> >>>     we use the qemu_get_fd() API.
> >>>
> >>>     Note: qemu_get_fd() is temporary, it shouldn't be a problem to use it
> >>>     (if it's not incorrect, of course, I honestly haven't fully tested it 
> >>> yet).
> >>
> >> So there will be a common super-class of Monitor and that new QMP
> >> session that also manages the file descriptors? That would make sense.
> > 
> > Oh, yes. Now I see that you said exactly that earlier. Sorry for more or 
> > less
> > re-stating it.
> > 
> >> Still, there would be monitor_get_fd and qmp_get_fd then not
> >> qemu_get_fd. I think that should be done already.
> > 
> > The problem is that monitor_get_fd() already exists and qmp_get_fd()
> > doesn't make much sense (as this is not related to QMP right now). So,
> > I could call it monitor_get_fd_cur() or something like this.
> 
> What object represent a QMP session now? 

We don't exactly have the notion of a QMP session today, but all QMP state
is currently stored in the Monitor object.

> That object will once hold the
> reference to the FDs. So some qmp_get_fd will take that session and
> return the requested fd - so, it does make sense, long-term at least.

Yes. Actually most of the code has already been written by Anthony:

 git://repo.or.cz/qemu/aliguori.git glib

 (look at qmp-core.c)

What I'm doing is to rebase it, do some integration work & fix ups.

> In any case, as long as everyone can mess with cur_mon, you don't need
> to introduce wrappers that just link a normal monitor service with that
> variable.

So, you're suggestion to just use monitor_get_fd(), right?