Re: [PATCH RFC 00/26] Multifd 🔀 device state transfer support with VFIO consumer

[Peter Xu]

On Fri, Apr 19, 2024 at 11:07:21AM +0100, Daniel P. Berrangé wrote:
> On Thu, Apr 18, 2024 at 04:02:49PM -0400, Peter Xu wrote:
> > On Thu, Apr 18, 2024 at 08:14:15PM +0200, Maciej S. Szmigiero wrote:
> > > I think one of the reasons for these results is that mixed (RAM + device
> > > state) multifd channels participate in the RAM sync process
> > > (MULTIFD_FLAG_SYNC) whereas device state dedicated channels don't.
> > 
> > Firstly, I'm wondering whether we can have better names for these new
> > hooks.  Currently (only comment on the async* stuff):
> > 
> >   - complete_precopy_async
> >   - complete_precopy
> >   - complete_precopy_async_wait
> > 
> > But perhaps better:
> > 
> >   - complete_precopy_begin
> >   - complete_precopy
> >   - complete_precopy_end
> > 
> > ?
> > 
> > As I don't see why the device must do something with async in such hook.
> > To me it's more like you're splitting one process into multiple, then
> > begin/end sounds more generic.
> > 
> > Then, if with that in mind, IIUC we can already split ram_save_complete()
> > into >1 phases too. For example, I would be curious whether the performance
> > will go back to normal if we offloading multifd_send_sync_main() into the
> > complete_precopy_end(), because we really only need one shot of that, and I
> > am quite surprised it already greatly affects VFIO dumping its own things.
> > 
> > I would even ask one step further as what Dan was asking: have you thought
> > about dumping VFIO states via multifd even during iterations?  Would that
> > help even more than this series (which IIUC only helps during the blackout
> > phase)?
> 
> To dump during RAM iteration, the VFIO device will need to have
> dirty tracking and iterate on its state, because the guest CPUs
> will still be running potentially changing VFIO state. That seems
> impractical in the general case.

We already do such interations in vfio_save_iterate()?

My understanding is the recent VFIO work is based on the fact that the VFIO
device can track device state changes more or less (besides being able to
save/load full states).  E.g. I still remember in our QE tests some old
devices report much more dirty pages than expected during the iterations
when we were looking into such issue that a huge amount of dirty pages
reported.  But newer models seem to have fixed that and report much less.

That issue was about GPU not NICs, though, and IIUC a major portion of such
tracking used to be for GPU vRAMs.  So maybe I was mixing up these, and
maybe they work differently.

Thanks,

-- 
Peter Xu