Re: [Qemu-devel] [PATCH] balloon: Fix failure of updating guest memory status

[Li, Liang Z]

> >> What if we are not in the just-after-live-migration situation though?
> >> If the guest simply didn't add a buffer to the queue for some reason,
> >> wouldn't this newly added push/notify break the balloon protocol?
> >>
> > Could you elaborate how it happens?
> > The added code only works for the situation just after live migration. 
> > right?
> 
> I don't believe so. As eloquently stated in the spec (5.5.6.3 Memory
> Statistics), the stats queue is "atypical". If everything goes well, the
> stats_vq_elem field is indeed expected to be non-NULL here in
> balloon_stats_poll_cb. But it may as well be NULL, for example if step 3. "The
> driver collects memory statistics and writes them into a new buffer." takes a
> long time and the driver doesn't make it by the time the callback fires.
> 
> Basically, the driver and device play ping-pong and the value of
> stats_vq_elem is NULL or non-NULL depending on which side the ball is.
> It looks like stats_vq_elem should be part of the QEMU state that is
> preserved across migrations, although I'll admit that I am unfamiliar with how

Migrate the stats_vq_elem to destination is better, and it can solve the 
similar problem
I encountered in another patch, I will look into how to do this.

Thanks for your explanation!

Liang

> migrations work and may be missing something important.
> 
> Thanks!
> Ladi
>