Re: [PATCH 9/9] contrib/plugins: add ips plugin example for cost modeling

[Alex Bennée]

Pierrick Bouvier <pierrick.bouvier@linaro.org> writes:

> On 6/18/24 02:53, Alex Bennée wrote:
>> Pierrick Bouvier <pierrick.bouvier@linaro.org> writes:
>> 
>>> On 6/17/24 13:56, Dr. David Alan Gilbert wrote:
>>>> * Pierrick Bouvier (pierrick.bouvier@linaro.org) wrote:
>>>>> On 6/14/24 15:00, Dr. David Alan Gilbert wrote:
>>>>>> * Pierrick Bouvier (pierrick.bouvier@linaro.org) wrote:
>>>>>>> Hi Dave,
>>>>>>>
>>>>>>> On 6/12/24 14:02, Dr. David Alan Gilbert wrote:
>>>>>>>> * Alex Bennée (alex.bennee@linaro.org) wrote:
>>>>>>>>> From: Pierrick Bouvier <pierrick.bouvier@linaro.org>
>>>>>>>>>
>>>>>>>>> This plugin uses the new time control interface to make decisions
>>>>>>>>> about the state of time during the emulation. The algorithm is
>>>>>>>>> currently very simple. The user specifies an ips rate which applies
>>>>>>>>> per core. If the core runs ahead of its allocated execution time the
>>>>>>>>> plugin sleeps for a bit to let real time catch up. Either way time is
>>>>>>>>> updated for the emulation as a function of total executed instructions
>>>>>>>>> with some adjustments for cores that idle.
>>>>>>>>
>>>>>>>> A few random thoughts:
>>>>>>>>       a) Are there any definitions of what a plugin that controls time
>>>>>>>>          should do with a live migration?
>>>>>>>
>>>>>>> It's not something that was considered as part of this work.
>>>>>>
>>>>>> That's OK, the only thing is we need to stop anyone from hitting problems
>>>>>> when they don't realise it's not been addressed.
>>>>>> One way might be to add a migration blocker; see 
>>>>>> include/migration/blocker.h
>>>>>> then you might print something like 'Migration not available due to 
>>>>>> plugin ....'
>>>>>>
>>>>>
>>>>> So basically, we could make a call to migrate_add_blocker(), when someone
>>>>> request time_control through plugin API?
>>>>>
>>>>> IMHO, it's something that should be part of plugin API (if any plugin 
>>>>> calls
>>>>> qemu_plugin_request_time_control()), instead of the plugin code itself. 
>>>>> This
>>>>> way, any plugin getting time control automatically blocks any potential
>>>>> migration.
>>>> Note my question asked for a 'any definitions of what a plugin ..' -
>>>> so
>>>> you could define it that way, another one is to think that in the future
>>>> you may allow it and the plugin somehow interacts with migration not to
>>>> change time at certain migration phases.
>>>>
>>>
>>> I would be in favor to forbid usage for now in this context. I'm not
>>> sure why people would play with migration and plugins generally at
>>> this time (there might be experiments or use cases I'm not aware of),
>>> so a simple barrier preventing that seems ok.
>>>
>>> This plugin is part of an experiment where we implement a qemu feature
>>> (icount=auto in this case) by using plugins. If it turns into a
>>> successful usage and this plugin becomes popular, we can always lift
>>> the limitation later.
>>>
>>> @Alex, would you like to add this now (icount=auto is still not
>>> removed from qemu), or wait for integration, and add this as another
>>> patch?
>> I think we follow the deprecation process so once integrated we post
>> a
>> deprecation notice in:
>>    https://qemu.readthedocs.io/en/master/about/deprecated.html
>> and then remove it after a couple of releases.
>> 
>
> Sorry, I was not clear. I meant do we add a blocker in case someone
> tries to migrate a vm while this plugin is used?

Oh yes - I can add that in the core plugin code.

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro