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

[Steven Sistare <steven.sistare@oracle.com>]

On 11/13/2024 4:58 PM, Peter Xu wrote:
> On Fri, Nov 01, 2024 at 06:47:50AM -0700, Steve Sistare wrote:
>> Add the cpr-transfer migration mode.  Usage:
>>    qemu-system-$arch -machine anon-alloc=memfd ...
>>
>>    start new QEMU with "-incoming <uri-1> -cpr-uri <uri-2>"
>>
>>    Issue commands to old QEMU:
>>    migrate_set_parameter mode cpr-transfer
>>    migrate_set_parameter cpr-uri <uri-2>
>>    migrate -d <uri-1>
> 
> QMP command "migrate" already allows taking MigrationChannel lists, cpr can
> be the 2nd supported channel besides "main".
> 
> I apologize on only noticing this until now.. I wished the incoming side
> can do the same already (which also takes 'MigrationChannel') if monitors
> init can be moved earlier, and if precreate worked out.  If not, we should
> still consider doing that on source, because cpr-uri isn't usable on dest
> anyway.. so they need to be treated separately even now.
> 
> Then after we make the monitor code run earlier in the future we could
> introduce that to incoming side too, obsoleting -cpr-uri there.

I have already been shot down on precreate and monitors init, so we are
left with specifying a "cpr" channel on the outgoing side, and -cpr-uri
on the incoming side.  That will confuse users, will require more implementation
and specification work than you perhaps realize to explain this to users,
and only gets us halfway to your desired end point of specifying everything
using channels.  I don't like that plan!

If we ever get the ability to open the monitor early, then we can implement
a complete and clean solution using channels and declare the other options
obsolete.

- Steve