Re: [PATCH v2 0/9] qapi: Use visitors for migration parameters handling

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> On Mon, Feb 02, 2026 at 07:40:52PM -0300, Fabiano Rosas wrote:
>> Hi, just a few more patches until options.c is fully using QAPI_CLONE
>> instead of open-coding options handling, we're almost there.
>
> While I'm reading your series, I found one thing that migration might be
> racy against for a while, and this series should enlarge that window of
> this issue: I don't think it's safe to free elements in MigrationParameters
> in the main thread, even if we hold BQL. Because migration thread can
> access parameters anytime without BQL... logically it can read any
> parameter, then if it's not a scalar we need some luck not crashing.
>

Good point, I had thought of it and convinced myself it was ok
somehow. I'll take another look.

> IMHO we may need to move faster on the "whitelist that can be dynamically
> set during migration" plan on migration parameters and capabilities.
>

Or we make set-parameters always work on top of temporary
MigrationParameter objects and switch the pointer atomically. Might be a
good idea regardless.

> I don't think any cap (after converted to parameters) should be on this
> whitelist... for parameters, I'm trying to be conservative but I believe
> the list should look like this:
>
> CPU throttle knobs:
>
>         throttle-trigger-threshold
>         cpu-throttle-increment
>         cpu-throttle-tailslow
>         max-cpu-throttle
>         x-vcpu-dirty-limit-period
>         vcpu-dirty-limit
>
> Bandwith knobs:
>
>         max-bandwidth
>         avail-switchover-bandwidth
>         downtime-limit
>         max-postcopy-bandwidth
>
> COLO knob:
>
>         x-checkpoint-delay
>
> That really should be all parameters we allow to change..  Luckily all of
> them are scalars, so it's fine to keep the current migrate_params_free()
> and logic to "free then update".
>
> I wonder if we should even consider having this whitelist alongside with
> your this series if we want to be extra safe, but I'll let you decide.. I'm
> also OK we do it later, but maybe we don't want it to be too late either.

Yeah.. I'll see what I can do. Thanks for the reviews this far!