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


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