On Wed, Mar 16, 2022 at 06:40:08PM +0800, Peter Xu wrote: > On Wed, Mar 16, 2022 at 09:59:28AM +0000, Daniel P. Berrangé wrote: > > 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 ! > > Ah. :) > > > > > 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. > > I very much agree on the whole concept of what you proposed, either on the > new negotiation phase itself, or the idea that with the negotiation phase > we can try to auto-enable some features we not used to. > > What I wanted to express is we can't enable either preempt mode or multifd > automatically from qemu even with them, because these two are quite special > IMHO in that qemu doesn't know whether the mgmt app can handle the multiple > socket pairs. Yes we could teach qemu to dynamically accept new "fd"s, but > again IMHO that still needs to be intervened by the mgmt app.
My proposal absolutely *can* let QEMU do that automatically, and that is one of the most important benefits of it. [quote] Introduce one *final-no-more-never-again-after-this* migration capability called "protocol-negotiation". When that capability is set, first declare that henceforth the migration transport is REQUIRED to support **multiple**, **bi-directional** channels. We might only use 1 TCP channel in some cases, but it declares our intent that we expect to be able to use as many channels as we see fit henceforth. [/quote] IOW, any management app that enabled 'protocol-negotiation' is explicitly declaring that it accepts the new requirements for support for multiple channels. An app which enabled 'protocol-negotiation' capability while only allowing 1 chanels is simply broken, because it would be violating the documented requirements for the capability. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
