Re: [PATCH] qemu: fix switchover-ack regression for old qemu

When enabling switchover-ack on qemu from libvirt, the .party value was set to both source and target; however, qemuMigrationParamsCheck() only takes that into account to validate that the remote side of the migration supports the flag if it is marked optional or auto/always on. In the case of switchover-ack, when enabled on only the dst and not the src, the migration will fail if the src qemu does not support switchover-ack, as the dst qemu will issue a switchover-ack msg: qemu/migration/savevm.c -> loadvm_process_command -> migrate_send_rp_switchover_ack(mis) -> migrate_send_rp_message(mis, MIG_RP_MSG_SWITCHOVER_ACK, 0, NULL) Since the src qemu doesn't understand messages with header_type == MIG_RP_MSG_SWITCHOVER_ACK, qemu will kill the migration with error: qemu-kvm: RP: Received invalid message 0x0007 length 0x0000 qemu-kvm: Unable to write to socket: Bad file descriptor Looking at the original commit [1] for optional migration capabilities, it seems that the spirit of optional handling was to enhance a given existing capability where possible. Given that switchover-ack exclusively depends on return-path, adding it as optional to that cap feels right.