[Qemu-devel] seamless migration with spice

[Yonit Halperin <yhalperi@redhat.com>]

Hi,

We would like to implement seamless migration for Spice, i.e., keeping 
the currently opened spice client session valid after migration.
Today, the spice client establishes the connection to the destination 
before migration starts, and when migration completes, the client's 
session is moved to the destination, but all the session data is being 
reset.

We face 2 main challenges when coming to implement seamless migration:

(1) Spice client must establish the connection to the destination before 
the spice password expires. However, during migration, qemu main loop is 
not processed, and when migration completes, the password might have 
already expired.

Today we solve this by the async command client_migrate_info, which is 
expected to be called before migration starts. The command is completed
once spice client has connected to the destination (or a timeout).

Since async monitor commands are no longer supported, we are looking for 
a new solution.
The straightforward solution would be to process the main loop on the 
destination side during migration.

(2) In order to restore the source-client spice session in the 
destination, we need to pass data from the source to the destination.
Example for such data: in flight copy paste data, in flight usb data
We want to pass the data from the source spice server to the 
destination, via Spice client. This introduces a possible race: after 
migration completes, the source qemu can be killed before the 
spice-server completes transferring the migration data to the client.

Possible solutions:
- Have an async migration state notifiers. The migration state will 
change after all the notifiers complete callbacks are called.
- libvirt will wait for qmp event corresponding to spice completing its 
migration, and only then will kill the source qemu process.

Any thoughts?

Thanks,
Yonit.