Re: [PATCH V3 11/16] migration: cpr-transfer mode

[Peter Xu <peterx@redhat.com>]

On Tue, Nov 19, 2024 at 04:41:07PM -0500, Steven Sistare wrote:
> On 11/19/2024 4:29 PM, Peter Xu wrote:
> > On Tue, Nov 19, 2024 at 04:03:08PM -0500, Steven Sistare wrote:
> > > On 11/19/2024 3:51 PM, Peter Xu wrote:
> > > > On Tue, Nov 19, 2024 at 03:32:55PM -0500, Steven Sistare wrote:
> > > > > This begs the question, should we allow channels to be specified in hmp migrate
> > > > > commands and for -incoming, in a very simple way?  Like with a prefix naming
> > > > > the channel.  And eliminate the -cpr-uri argument. Examples:
> > > > > 
> > > > > (qemu) migrate -d main:tcp:0:44444,cpr:unix:cpr.sock
> > > > > 
> > > > > qemu -incoming main:tcp:0:44444,cpr:unix:cpr.sock
> > > > > qemu -incoming main:defer,cpr:unix:cpr.sock
> > > > 
> > > > IMHO keeping the old syntax working would still be nice to not break
> > > > scripts.
> > > 
> > > The channel tag would be optional, so backwards compatible.  Its unambiguous
> > > as long as the channel names are not also protocol names.
> > 
> > Ah that's ok then.  Or maybe use "="?
> > 
> >    "main=XXX,cpr=XXX"
> > 
> > Then if no "=" it's the old?
> 
> Sure, that works.
> 
> > > > I was thinking we could simply add one more parameter for taking
> > > > cpr uri, like:
> > > > 
> > > >       {
> > > >           .name       = "migrate",
> > > >           .args_type  = "detach:-d,resume:-r,uri:s,cpr:s?",
> > > >           .params     = "[-d] [-r] uri [cpr_uri]",
> > > >           .help       = "migrate to URI (using -d to not wait for completion)"
> > > > 		      "\n\t\t\t -r to resume a paused postcopy migration",
> > > > 		      "\n\t\t\t Setup cpr_uri to migrate with cpr-transfer",
> > > >           .cmd        = hmp_migrate,
> > > >       },
> > > 
> > > That's fine.
> > > 
> > > I do like the incoming syntax, though, instead of -cpr-uri.  What do you think?
> > 
> > That'll definitely be lovely if possible, though would any monitor be alive
> > at all before taking a cpr stream, with this series alone?  I thought you
> > dropped the precreate, then QEMU isn't able to run the monitor loop until
> > cpr-uri is loaded.
> 
> No monitor or precreate changes.  I would parse -incoming, extract and use the cpr
> channel early, and use the main channel later as usual.  It's just a different way of
> specifying cpr-uri.  I like it because the specification language is more consistent,
> referring to a "cpr channel" both on the outgoing and incoming side:
> 
>   This mode requires a second migration channel named "cpr", included in
>   the channel arguments of the migrate command on the outgoing side, and
>   in the QEMU -incoming parameter on the incoming side.  The channel must
>   be a type, such as unix socket, that supports SCM_RIGHTS.

Ah, that's ok at least to me.  I hope defer could still work (for Libvirt),
though.  Probably something like main=defer,cpr=XXX.

-- 
Peter Xu