[libvirt] Tunnelled post-copy live migration - support of bidirectional communication over the tunnel

[Vojtech Cima]

Hello everyone,

we are very interested about the post-copy live migration mechanism in 
libvirt [1] using QEMU. Mentioning post-copy in this context means the 
setup with the first full iteration of standard pre-copy and then 
switching to the post-copy. VIR_MIGRATE_ENABLE_POSTCOPY, 
VIR_MIGRATE_POSTCOPY_AFTER_PRECOPY together with VIR_MIGRATE_TUNNELLED flag.

Recently we have noticed the issue that post-copy live migration doesn't 
work over the tunnel since since current tunnel implementation in 
libvirt supports only uni-directional communication. I spent some time 
trying to enable bidirectional communication over the tunnel but still 
facing several issues and I would appreciate any hints how to move forward.

The idea is to add a function (probably a callback function) responsible 
for handling the traffic in the opposite direction (dest->src) reading 
from the virStream and writing to the QEMU [2]. To be able to do so I 
replaced the current 'pipe' calls with the 'socketpair' calls to create 
bidirectional paths.

Now, trying to use tunnelled post-copy live migration, it gets 
initiated, there is some traffic going from source to destination 
(obviously the pre-copy phase) but at some point it gets stuck on the 
'saferead' function [3] which is now blocking. Can this issue issue be 
linked with the default socket blocking policy?
Trying to use standard 'read' function instead of 'saferead' results 
that the lower data transfers and it seems it's missing significant part 
of information.

Another point is that we suppose that the virStream on its own supports 
bidirectional communication, is that correct?

[1] https://github.com/orbitfp7/qemu/tree/wp3-postcopy

[2] 
http://libvirt.org/git/?p=libvirt.git;a=blob;f=src/qemu/qemu_migration.c;h=0acbb5795e930e27157c580a3236701acd91bbaf;hb=e3435caf6af41748204e542dee13ede8441d88c0#l3595

[3] 
http://libvirt.org/git/?p=libvirt.git;a=blob;f=src/qemu/qemu_migration.c;h=0acbb5795e930e27157c580a3236701acd91bbaf;hb=e3435caf6af41748204e542dee13ede8441d88c0#l3644

Thank you very much for any ideas on this topic.

Regards,
Vojtech