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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-02-15 18:23, Luiz Capitulino wrote:
> On Wed, 15 Feb 2012 14:34:52 +0100
> Jan Kiszka <jan.kiszka@siemens.com> wrote:
> 
>> On 2012-02-15 13:49, Luiz Capitulino wrote:
>>> On Wed, 15 Feb 2012 09:59:07 +0100
>>> Jan Kiszka <jan.kiszka@siemens.com> 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)

> 
>>>
>>>  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? 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.

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.

Jan

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