Re: [Qemu-devel] live migration vs device assignment (motivation)

[Michael S. Tsirkin]

On Mon, Dec 28, 2015 at 03:20:10AM +0000, Dong, Eddie wrote:
> > >
> > > Even if the device driver doesn't support migration, you still want to
> > > migrate VM? That maybe risk and we should add the "bad path" for the
> > > driver at least.
> > 
> > At a minimum we should have support for hot-plug if we are expecting to
> > support migration.  You would simply have to hot-plug the device before you
> > start migration and then return it after.  That is how the current bonding
> > approach for this works if I am not mistaken.
> 
> Hotplug is good to eliminate the device spefic state clone, but
> bonding approach is very network specific, it doesn’t work for other
> devices such as FPGA device, QaT devices & GPU devices, which we plan
> to support gradually :)

Alexander didn't say do bonding. He just said bonding uses hot-unplug.

Gradual and generic is the correct approach. So focus on splitting the
work into manageable pieces which are also useful by themselves, and
generally reusable by different devices.

So live the pausing alone for a moment.

Start from Alexander's patchset for tracking dirty memory, add a way to
control and detect it from userspace (and maybe from host), and a way to
start migration while device is attached, removing it at the last
possible moment.

That will be a nice first step.


> > 
> > The advantage we are looking to gain is to avoid removing/disabling the
> > device for as long as possible.  Ideally we want to keep the device active
> > through the warm-up period, but if the guest doesn't do that we should still
> > be able to fall back on the older approaches if needed.
> >