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

[Pierrick Bouvier]

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?