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

[Alex Bennée]

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.

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro