Re: [Qemu-devel] [PATCH v3 19/50] tcg: let plugins instrument memory accesses

[Aaron Lindsay OS via Qemu-devel <qemu-devel@nongnu.org>]

On Jul 01 16:00, Alex Bennée wrote:
> Aaron Lindsay OS <aaron@os.amperecomputing.com> writes:
> > - a way for a plugin to reset any instrumentation decisions made in the
> >   past (essentially calls `tb_flush(cpu);` under the covers). We found
> >   this critical for plugins which undergo state changes during the
> >   course of their execution (i.e. watch for event X, then go into a more
> >   detailed profiling mode until you see event Y)
> 
> check:
> 
> /**
>  * qemu_plugin_reset() - Reset a plugin
>  * @id: this plugin's opaque ID
>  * @cb: callback to be called once the plugin has been reset
>  *
>  * Unregisters all callbacks for the plugin given by @id.
>  *
>  * Do NOT assume that the plugin has been reset once this function returns.
>  * Plugins are reset asynchronously, and therefore the given plugin receives
>  * callbacks until @cb is called.
>  */
> void qemu_plugin_reset(qemu_plugin_id_t id, qemu_plugin_simple_cb_t cb);

Is this essentially synchronous for the current cpu, and only
asynchronous for any other running cpus that didn't trigger the callback
from which the call to qemu_plugin_reset() is being made? If not, could
the state resetting be made synchronous for the current cpu (even if the
callback doesn't happen until the others are complete)? This isn't
absolutely critical, but it is often nice to begin capturing precisely
when you mean to.

> > - the ability for a plugin to trigger a checkpoint to be taken
> 
> We don't have this at the moment. Pranith also mentioned it in his
> review comments. I can see its use but I suspect it won't make the
> initial implementation given the broader requirements of QEMU to do
> checkpointing and how to cleanly expose that to plugins.

Sure. Our patch works for us, but I know we're ignoring a few things
that we can externally ensure won't happen while we're attempting a
checkpoint (i.e. migration) that may have to be considered for something
upstream.

-Aaron