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    SubjectRe: [PATCH] virtio-balloon spec: provide a version of the "silent deflate" feature that works
    On Thu, Sep 06, 2012 at 09:46:50AM +0200, Paolo Bonzini wrote:
    > VIRTIO_BALLOON_F_MUST_TELL_HOST cannot be used properly because it is a
    > "negative" feature: it tells you that silent defalte is not supported.
    > Right now, QEMU refuses migration if the target does not support all the
    > features that were negotiated. But then:
    >
    > - a migration from non-MUST_TELL_HOST to MUST_TELL_HOST will succeed,
    > which is wrong;
    >
    > - a migration from MUST_TELL_HOST to non-MUST_TELL_HOST will fail, which
    > is useless.
    >
    > Add instead a new feature VIRTIO_BALLOON_F_SILENT_DEFLATE, and deprecate
    > VIRTIO_BALLOON_F_MUST_TELL_HOST since it is never actually used.
    >
    > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

    Frankly I think it's a qemu migration bug. I don't see why
    we need to tweak spec: just teach qemu to be smarter
    during migration.

    Can you show a scenario with old driver/new hypervisor or
    new driver/old hypervisor that fails?

    > ---
    > virtio-spec.lyx | 36 +++++++++++++++++++++++++++++++++---
    > 1 file modificato, 33 inserzioni(+), 3 rimozioni(-)
    >
    > diff --git a/virtio-spec.lyx b/virtio-spec.lyx
    > index 7a073f4..1a25a18 100644
    > --- a/virtio-spec.lyx
    > +++ b/virtio-spec.lyx
    > @@ -6238,6 +6238,8 @@ bits
    >
    > \begin_deeper
    > \begin_layout Description
    > +
    > +\change_deleted 1531152142 1346917221
    > VIRTIO_BALLOON_F_MUST_TELL_HOST
    > \begin_inset space ~
    > \end_inset
    > @@ -6251,6 +6253,20 @@ VIRTIO_BALLOON_F_STATS_VQ
    > \end_inset
    >
    > (1) A virtqueue for reporting guest memory statistics is present.
    > +\change_inserted 1531152142 1346917193
    > +
    > +\end_layout
    > +
    > +\begin_layout Description
    > +
    > +\change_inserted 1531152142 1346917219
    > +VIRTIO_BALLOON_F_SILENT_DEFLATE
    > +\begin_inset space ~
    > +\end_inset
    > +
    > +(2) Host does not need to be told before pages from the balloon are used.
    > +\change_unchanged
    > +
    > \end_layout
    >
    > \end_deeper
    > @@ -6401,9 +6417,23 @@ The driver constructs an array of addresses of memory pages it has previously
    > \end_layout
    >
    > \begin_layout Enumerate
    > -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.
    > +If the VIRTIO_BALLOON_F_
    > +\change_deleted 1531152142 1346917234
    > +MUST_TELL_HOST
    > +\change_inserted 1531152142 1346917237
    > +SILENT_DEFLATE
    > +\change_unchanged
    > + feature is
    > +\change_inserted 1531152142 1346917241
    > +not
    > +\change_unchanged
    > +set, the guest may not use these requested pages until that descriptor in
    > + the deflateq has been used by the device.
    > +
    > +\change_inserted 1531152142 1346917253
    > + If it is set, the guest may choose to not use the deflateq at all.
    > +\change_unchanged
    > +
    > \end_layout
    >
    > \begin_layout Enumerate
    > --
    > 1.7.11.2


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