Re: [PATCH v3 5/7] implementation of vm_start() BH

[Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>]

On 19.11.2020 21:46, Peter Xu wrote:
> 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.
> 

Yes, agree, better to squash this small commit.

-- 
Andrey Gruzdev, Principal Engineer
Virtuozzo GmbH  +7-903-247-6397
                 virtuzzo.com