Re: [PATCH] vfio/migrate: Move switch of dirty tracking into vfio_memory_listener

[Paolo Bonzini]

On 28/01/21 21:02, Dr. David Alan Gilbert wrote:
> * Paolo Bonzini (pbonzini@redhat.com) wrote:
> > On 11/01/21 08:34, Keqian Zhu wrote:
> > > +static void vfio_listener_log_start(MemoryListener *listener,
> > > +                                    MemoryRegionSection *section,
> > > +                                    int old, int new)
> > > +{
> > > +    VFIOContainer *container = container_of(listener, VFIOContainer, listener);
> > > +
> > > +    vfio_set_dirty_page_tracking(container, true);
> > > +}
> >
> > This would enable dirty page tracking also just for having a framebuffer
> > (DIRTY_MEMORY_VGA).  Technically it would be correct, but it would also be
> > more heavyweight than expected.
>
> Wouldn't that only happen on emulated video devices?

Yes, but still it's not impossible to have both an emulated VGA and an 
assigned GPU or vGPU.

> > In order to only cover live migration, you can use the log_global_start and
> > log_global_stop callbacks instead.
> >
> > If you want to use log_start and log_stop, you need to add respectively
> >
> >      if (old != 0) {
> >          return;
> >      }
> >
> > and
> >
> >      if (new != 0) {
> >          return;
> >      }
>
> Why 0, wouldn't you be checking for DIRTY_LOG_MIGRATION somewhere?

Actually thinking more about it log_start/log_stop are just wrong, 
because they would be called many times, for each MemoryRegionSection.

Paolo