[RFC v2 0/3] migration: reduce time of loading non-iterable vmstate

[Chuang Xu]

Hi!

In this version:

- rebase to latest upstream.
- add sanity check to address_space_to_flatview().
- postpone the init of the vring cache until migration's loading completes. 

Please review, Chuang.

[v1]

The duration of loading non-iterable vmstate accounts for a significant
portion of downtime (starting with the timestamp of source qemu stop and
ending with the timestamp of target qemu start). Most of the time is spent
committing memory region changes repeatedly.

This patch packs all the changes to memory region during the period of
loading non-iterable vmstate in a single memory transaction. With the
increase of devices, this patch will greatly improve the performance.

Here are the test results:
test vm info:
- 32 CPUs 128GB RAM
- 8 16-queue vhost-net device
- 16 4-queue vhost-user-blk device.

        time of loading non-iterable vmstate
before          about 210 ms
after           about 40 ms