Re: [RFC] New thread for the VM migration

[Juan Quintela <quintela@redhat.com>]

Avi Kivity <avi@redhat.com> wrote:
> On 07/14/2011 03:30 PM, Anthony Liguori wrote:
>>> Does this mean that the following code is sometimes executed without
>>> qemu_mutex? I don't think any of it is thread safe.
>>
>>
>> That was my reaction too.
>>
>> I think the most rational thing to do is have a separate thread and
>> a pair of producer/consumer queues.
>>
>> The I/O thread can push virtual addresses and sizes to the queue for
>> the migration thread to compress/write() to the fd.  The migration
>> thread can then push sent regions onto a separate queue for the I/O
>> thread to mark as dirty.
>
> Even virtual addresses are not safe enough, because of hotunplug.
> Without some kind of locking, you have to copy the data.

Disabling hotplug should be enough? Notice that hotplug/unplug during
migration don't make a lot of sense anyways.

Not all the bitmap syncying has proper locking now (copyng towards one
place), but rest of cade looks really thread safe to me (migration code
is only called from this thread, so it should be safe).

My understanding on how this work:

 vcpu thread modifies momery
 iothread (some times) modifies memory

 migration thread: reads memory, and gets the lock before syncing its
 bitmap with kvm one and qemu one (clearing it on the time).

Assume we disable hotplug/unplug (what we have to do anyways).  What is
the locking problem that we have?

We do stage 3 with the iothread locked, i.e. at that point everything
else is stopped.  Before stage 3, can kvm or qemu modify a page and
_not_ modify the bitmap?  My understanding is not.

Only real variable that we are sharing is ram_list, or I am losing
something obvious?

Later, Juan.