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

[Maciej S. Szmigiero]

On 24.04.2024 00:20, Peter Xu wrote:
> On Tue, Apr 23, 2024 at 06:15:35PM +0200, Maciej S. Szmigiero wrote:
> > On 19.04.2024 17:31, Peter Xu wrote:
> > > 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.
> >
> > The device which this series was developed against (Mellanox ConnectX-7)
> > is already transferring its live state before the VM gets stopped (via
> > save_live_iterate SaveVMHandler).
> >
> > It's just that in addition to the live state it has more than 400 MiB
> > of state that cannot be transferred while the VM is still running.
> > And that fact hurts a lot with respect to the migration downtime.
> >
> > AFAIK it's a very similar story for (some) GPUs.
>
> So during iteration phase VFIO cannot yet leverage the multifd channels
> when with this series, am I right?

That's right.

> Is it possible to extend that use case too?

I guess so, but since this phase (iteration while the VM is still
running) doesn't impact downtime it is much less critical.
 
> Thanks,

Thanks,
Maciej