On Thu, Nov 19, 2020 at 03:59:38PM +0300, Andrey Gruzdev wrote:
To avoid saving updated versions of memory pages we need
to start tracking RAM writes before we resume operation of
vCPUs. This sequence is especially critical for virtio device
backends whos VQs are mapped to main memory and accessed
directly not using MMIO callbacks.
One problem is that vm_start() routine makes calls state
change notifier callbacks directly from itself. Virtio drivers
do some stuff with syncing/flusing VQs in its notifier routines.
Since we poll UFFD and process faults on the same thread, that
leads to the situation when the thread locks in vm_start()
if we try to call it from the migration thread.
There's a nice comment in previous patch about this before the bottom half
created, thanks, that's helpful. Though IMHO this patch can directly be
squashed into previous one, since it's confusing with the comment there but
without doing anything about it.