[PATCH] virtio-balloon spec: provide a version of the "silent deflate" feature that works

[Paolo Bonzini]

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@xxxxxxxxxx>
---
 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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