[libvirt] [RFC] make virDomainQemuMonitorCommand work in any libvirt state

[Nikolay Shirokovskiy]

On 19.10.2016 10:37, Daniel P. Berrange wrote:
> On Wed, Oct 19, 2016 at 09:50:37AM +0800, Michal Privoznik wrote:
>> On 17.10.2016 20:46, Nikolay Shirokovskiy wrote:
>>> Hi, all.
>>>
>>> We would like to use virDomainQemuMonitorCommand to query qemu independently of
>>> libvirt state. Currenly it is not possible. This API call takes job condition
>>> just like any other call and thus is unavailable on any lengthy(or stucked)
>>> synchronous job.
>>>
>>> I've already posted this question in list, just failed to find the reference.
>>> Somebody suggested to use proxy (and even an implementation) in between qemu
>>> and libvirt that can inject commands to qemu and filter replies. It is not
>>> really convinient. This way test setups will be different from production and
>>> we can not investigate problems in production environment.
>>>
>>> I'd like to drop acquiring job condition in the call as this function does not
>>> deal with libvirt state (except for the taint but is is ok, we will not mess
>>> things up here). But this is not enough, we need to make qemu monitor deal with
>>> many qemu commands simultaneously. Looks like it is quite a big change for
>>> test/debug case. But I guess eventually normal user cases can get benefits too
>>> from this monitor changes. For example all query API calls that query qemu
>>> directly can be changed to not to wait for some synchronous job
>>> finishing.(qemuDomainGetBlockJobInfo for example).
>>
>> IIRC the last time I looked into this the problem was not on libvirt
>> side. QEMU's monitor was unable to process multiple commands at once.
>> But maybe that's no longer the case, I don't know.
> 
> That is still the case. If you send a command while something else
> is running QEMU will not process it until the previous command is
> completed.

Ok. Then now is not appropriate time to address the problem in libvirt.

Denis mentioned another case when libvirt is too restrictive on
parallel execution. API calls to agent and qemu are serialized
while at the first glance they not have to. We would like to 
introduce different job condition for agent API calls. And if
agent can process multiple commands at once we can change agent
monitor to issue multiple commands too.

> 
> There was a proposal last week to address this in QEMU, but it is
> not going to be fully fixed anytime soon.
> 
>> However, what I think we should do is to turn our jobs into sort of RW
>> locks. That is - we could allow multiple QUERY jobs to happen
>> simultaneously and leave MODIFY jobs to be exclusive. I think dropping
>> BeginJob() from an API is a no go as it will definitely bite us in the
>> future.
>>
>> Unfortunately, I have no idea what my suggestion would look like in
>> terms of the code. How difficult it would be to implement it (and
>> whether monitor code is prepared for that).
> 

Nikolay