Re: Time to introduce a migration protocol negotiation (Re: [PATCH v2 00/25] migration: Postcopy Preemption)

[Daniel P . Berrangé]

On Wed, Mar 16, 2022 at 11:30:59AM +0800, Peter Xu wrote:
> On Tue, Mar 15, 2022 at 11:15:41AM +0000, Daniel P. Berrangé wrote:
> > > I still remember you mentioned the upper layer softwares can have
> > > assumption on using only 1 pair of socket for migration, I think that 
> > > makes
> > > postcopy-preempt by default impossible.
> > > 
> > > Why multifd is different here?
> > 
> > It isn't different. We went through the pain to extending libvirt
> > to know how to open many channels for multifd. We'll have todo
> > the same with this postcopy-pre-empt. To this day though, management
> > apps above libvirt largely don't enable multifd, which is a real
> > shame. This is the key reason I think we need to handle this at
> > the QEMU level automatically.
> 
> But I still don't undertand how QEMU could know about those tunnels, which
> should be beyond QEMU's awareness?
> 
> The tunneling program can be some admin initiated socat tcp forwarding
> programs, which by default may not allow >1 socket pairs.
> 
> Or maybe I have mis-understood on what's the tunneling we're discussing?

I dont think I was talking about tunneling at all, just QEMU
migration protocol options !

If an app is tunnelling QEMU's migration protocol over some
channel, that isn't important to QEMU - regardless whether a
passed in 'fd:' protocol FD is a direct TCP socket, or a
UNIX socket for a tunnel, QEMU works the same way. In one
of my other replies I mention a way to make 'fd:' work with
an arbitrary number of channels, by using an event from QEMU
to request the app provide additional FDs.


With regards,
Daniel
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