Re: [PATCH] virtio_balloon: Notify guest only after deflating the balloon

[Rusty Russell <rusty@rustcorp.com.au>]

On Sun, 3 Jul 2011 15:11:37 +0300, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> On Sun, Jul 03, 2011 at 01:52:29PM +0300, Sasha Levin wrote:
> > This just leads me to believe that we should either not notify the host,
> > or not wait_for_completion() when telling the host.
> 
> Interesting. The spec says
> 
> (a) The driver constructs an array of addresses of memory pages it has
> previously given to the balloon, as described above. This descriptor
> is added to the deflateq.
> (b) If the VIRTIO_BALLOON_F_MUST_TELL_HOST feature is set,
> the guest may not use these requested pages until that descriptor in
> the deflateq has been used by the device.
> (c) Otherwise, the guest may begin to re-use pages previously given to
> the balloon before the device has acknowledged their withdrawl.
>  21 In this case, deflation advice is merely a courtesy
> 
> This does not discuss the following issue: what happens
> if the device never uses the descriptor in the deflateq
> and VIRTIO_BALLOON_F_MUST_TELL_HOST is not set?
> 
> Rusty, any comments?

The device still *has* a queue.  It should process it!

The feature should be called VIRTIO_BALLOON_F_ASK_HOST_BEFORE_REUSE.

If we didn't want the information at *all*, we would have made the queue
not exist in that case...

Thanks,
Rusty.