Il 08/05/2013 12:10, Paolo Bonzini ha scritto:
> The idea of the VIRTIO_BALLOON_F_MUST_TELL_HOST feature is to let drivers
> skip usage of the deflate queue when leaking the balloon ("silent
> deflation"). Guests may benefit from silent deflate by aggressively
> inflating the balloon; they know that they will be able to use ballooned
> pages without issuing a (blocking) request to the device.
>
> The problem is that this feature is a "negative" feature: if
> set, the guest _may not_ use ballooned pages directly. Negative features
> are not safe against migration; here is an explanation why this is so.
>
> For a "positive" feature, migration is possible if the destination
> supports it, or the source didn't set it:
>
> dest support source set ok?
> T T T
> T F T
> F T F
> F F T
>
> For a "negative" feature, migration is possible if the destination
> supports it, or the source set it:
>
> dest support source set ok?
> T T T
> T F F
> F T T
> F F T
>
> However, the F/T line violates the virtio specification because the
> negotiated features are supposed to be the AND of the device-
> and driver-supported features.
>
> Furthermore, this assumes that the destination host knows which features
> are "positive" and which are "negative", which obviously cannot be the
> case in general. (The original spec assumed that every device supports
> VIRTIO_BALLOON_F_MUST_TELL_HOST, but this was not explicitly documented
> and in practice it turns out not to be the case).
>
> Not all is lost, however. First, all known device implementations support
> silent deflation, hence they do not negotiate the feature. We are thus
> somewhat free to redefine what the host should do about this feature.
>
> Second, by chance, coincidence or an evil plot, the only known driver
> that does not negotiate VIRTIO_BALLOON_F_MUST_TELL_HOST is also using
> pages before telling the host. Thus, even though the feature used to be
> just for communication from the host, known drivers are really using it
> to communicate was in the other direction, as if the feature was named
> "VIRTIO_BALLOON_F_GUEST_TELLS_HOST".
>
> Adjust the spec to conform, and add a new feature bit for the host to
> tell the drivers if silent deflation is actually supported. With this
> new feature bit, the host can distinguish all three cases: will never
> do silent deflation, will do silent deflation if available, will always
> do silent deflation (as in the above buggy driver).
>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Ping?
Paolo
> ---
> virtio-spec.lyx | 264 ++++++++++++++++++++++++++++++++++++++++++++++++++++++--
> 1 file changed, 258 insertions(+), 6 deletions(-)
>
> diff --git a/virtio-spec.lyx b/virtio-spec.lyx
> index 73e22e7..033362f 100644
> --- a/virtio-spec.lyx
> +++ b/virtio-spec.lyx
> @@ -63,7 +63,7 @@
> \author -385801441 "Cornelia Huck" cornelia.huck@xxxxxxxxxx
> \author 460276516 "Dmitry Fleytman" dfleytma@xxxxxxxxxx
> \author 1112500848 "Rusty Russell" rusty@xxxxxxxxxxxxxxx
> -\author 1531152142 "Paolo Bonzini,,,"
> +\author 1531152142 "Paolo Bonzini" pbonzini@xxxxxxxxxx
> \author 1717892615 "Alexey Zaytsev,,,"
> \author 1986246365 "Michael S. Tsirkin"
> \end_header
> @@ -7179,11 +7179,49 @@ bits
>
> \begin_deeper
> \begin_layout Description
> -VIRTIO_BALLOON_F_MUST_TELL_HOST
> +VIRTIO_BALLOON_F_
> +\change_deleted 1531152142 1347020601
> +MUST
> +\change_inserted 1531152142 1347020602
> +GUEST
> +\change_unchanged
> +_TELL
> +\change_inserted 1531152142 1368004486
> +S
> +\change_unchanged
> +_HOST
> \begin_inset space ~
> \end_inset
>
> -(0) Host must be told before pages from the balloon are used.
> +(0)
> +\change_deleted 1531152142 1347020625
> +Host must be told
> +\change_inserted 1531152142 1347020617
> +Guest will tell host
> +\change_unchanged
> + before pages from the balloon are used.
> +
> +\change_inserted 1531152142 1368005603
> + The host should always propose this feature.
> +\begin_inset Foot
> +status open
> +
> +\begin_layout Plain Layout
> +
> +\change_inserted 1531152142 1347022389
> +This feature used to be named VIRTIO_BALLOON_F_\SpecialChar \-
> +MUST_TELL_HOST.
> + However, after a few years it was observed that drivers were not using
> + it as specified.
> + The virtio-balloon spec was then adjusted to what the drivers had been
> + doing.
> +\end_layout
> +
> +\end_inset
> +
> +
> +\change_unchanged
> +
> \end_layout
>
> \begin_layout Description
> @@ -7192,6 +7230,20 @@ VIRTIO_BALLOON_F_STATS_VQ
> \end_inset
>
> (1) A virtqueue for reporting guest memory statistics is present.
> +\change_inserted 1531152142 1347020627
> +
> +\end_layout
> +
> +\begin_layout Description
> +
> +\change_inserted 1531152142 1347020648
> +VIRTIO_BALLOON_F_SILENT_DEFLATE
> +\begin_inset space ~
> +\end_inset
> +
> +(2) Guest does not need to tell host before pages from the balloon are used.
> +\change_unchanged
> +
> \end_layout
>
> \end_deeper
> @@ -7342,9 +7394,27 @@ 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
> +\change_inserted 1531152142 1347025663
> +the VIRTIO_BALLOON_F_\SpecialChar \-
> +GUEST_\SpecialChar \-
> +TELLS_\SpecialChar \-
> +HOST feature is set, and
> +\change_unchanged
> +the VIRTIO_BALLOON_F_
> +\change_deleted 1531152142 1347021706
> +MUST_TELL_HOST
> +\change_inserted 1531152142 1347022257
> +\SpecialChar \-
> +SILENT_\SpecialChar \-
> +DEFLATE
> +\change_unchanged
> + feature is
> +\change_inserted 1531152142 1347025674
> +not
> +\change_unchanged
> +set, the guest may not use these requested pages until that descriptor in
> + the deflateq has been used by the device.
> \end_layout
>
> \begin_layout Enumerate
> @@ -7535,6 +7605,188 @@ VIRTIO_BALLOON_S_MEMFREE The amount of memory not being used for any purpose
> VIRTIO_BALLOON_S_MEMTOT The total amount of memory available (in bytes).
> \end_layout
>
> +\begin_layout Description
> +
> +\change_inserted 1531152142 1368005515
> +Silent
> +\begin_inset space ~
> +\end_inset
> +
> +deflation
> +\end_layout
> +
> +\begin_layout Standard
> +
> +\change_inserted 1531152142 1368005515
> +
> +\series medium
> +Some implementation of the balloon device may not require the guest to deflate
> + the balloon explicitly; instead, the guest may just take a page from its
> + reserve and start using it.
> + This is called
> +\begin_inset Quotes eld
> +\end_inset
> +
> +silent deflate
> +\begin_inset Quotes erd
> +\end_inset
> +
> +
> +\end_layout
> +
> +\begin_layout Standard
> +
> +\change_inserted 1531152142 1368005515
> +In order to use this feature effectively, both the guest and the host need
> + to know how the other part intends to use the balloon.
> +\end_layout
> +
> +\begin_layout Standard
> +
> +\change_inserted 1531152142 1368005515
> +
> +\series medium
> +Guests may benefit from silent deflate by aggressively inflating the balloon;
> + they know that they will be able to use ballooned pages without issuing
> + a (blocking) request to the device.
> +\end_layout
> +
> +\begin_layout Standard
> +
> +\change_inserted 1531152142 1368005515
> +Knowing that the guest will
> +\emph on
> +not
> +\emph default
> + deflate silently also benefits the host.
> + For example, if the host is pinning the guest's memory, it may unpin ballooned
> + pages and pin them again upon deflation.
> + This allows cooperative memory overcommit even if the guest's memory is
> + pinned.
> +\end_layout
> +
> +\begin_layout Standard
> +
> +\change_inserted 1531152142 1368005515
> +Thus, there are two possibilities for the host (either it does not support
> + silent deflate, or it does), and three for the guest (it doesn't need silent
> + deflate, it may choose to use it if available, it requires it).
> + Because there are three possibilities for the guest, support for silent
> + deflate is represented by two different feature bits.
> +\end_layout
> +
> +\begin_layout Standard
> +
> +\change_inserted 1531152142 1368005515
> +The feature bits are used as follows:
> +\end_layout
> +
> +\begin_layout Itemize
> +
> +\change_inserted 1531152142 1368005671
> +the host will always negotiate the VIRTIO_\SpecialChar \-
> +BALLOON_F_\SpecialChar \-
> +GUEST_\SpecialChar \-
> +TELLS_\SpecialChar \-
> +HOST feature.
> + If the guest does
> +\emph on
> +not
> +\emph default
> + negotiate it, the host should assume that the guest will use silent deflate.
> + If the host does not support silent deflate, it must not let the guest
> + discover any virtqueue, so that the driver will fail to start.
> +\end_layout
> +
> +\begin_layout Itemize
> +
> +\change_inserted 1531152142 1368005515
> +a guest that
> +\emph on
> +must
> +\emph default
> + use silent deflation does not need to negotiate any feature.
> +\end_layout
> +
> +\begin_layout Itemize
> +
> +\change_inserted 1531152142 1368005676
> +a guest that
> +\emph on
> +will never
> +\emph default
> + use silent deflation only has to propose VIRTIO_\SpecialChar \-
> +BALLOON_F_\SpecialChar \-
> +GUEST_\SpecialChar \-
> +TELLS_\SpecialChar \-
> +HOST.
> + It need not do anything if the host does not negotiate the feature.
> +\end_layout
> +
> +\begin_layout Itemize
> +
> +\change_inserted 1531152142 1368005686
> +a guest that
> +\emph on
> +can optionally
> +\emph default
> + use silent deflation should propose both VIRTIO_\SpecialChar \-
> +BALLOON_F_\SpecialChar \-
> +GUEST_\SpecialChar \-
> +TELLS_\SpecialChar \-
> +HOST
> + and VIRTIO_\SpecialChar \-
> +BALLOON_F_\SpecialChar \-
> +SILENT_\SpecialChar \-
> +DEFLATE.
> + The guest driver can then use silent deflation if and only if the host
> + has negotiated VIRTIO_\SpecialChar \-
> +BALLOON_F_\SpecialChar \-
> +SILENT_\SpecialChar \-
> +DEFLATE too.
> +\end_layout
> +
> +\begin_layout Standard
> +
> +\change_inserted 1531152142 1368005690
> +Old hosts may fail to propose the VIRTIO_\SpecialChar \-
> +BALLOON_F_\SpecialChar \-
> +GUEST_\SpecialChar \-
> +TELLS_\SpecialChar \-
> +HOST feature.
> + This is not a problem as long as these old hosts support silent deflation;
> + when running on such a host:
> +\end_layout
> +
> +\begin_layout Itemize
> +
> +\change_inserted 1531152142 1368005515
> +a guest that
> +\emph on
> +must
> +\emph default
> + use, or
> +\emph on
> +will never
> +\emph default
> + use silent deflation will just work.
> +\end_layout
> +
> +\begin_layout Itemize
> +
> +\change_inserted 1531152142 1368005691
> +a guest that
> +\emph on
> +can optionally
> +\emph default
> + use silent deflation will typically not use silent deflation, because these
> + old hosts do not know about VIRTIO_\SpecialChar \-
> +BALLOON_F_\SpecialChar \-
> +SILENT_\SpecialChar \-
> +DEFLATE, but the guest
> + will otherwise work normally.
> +\end_layout
> +
> \begin_layout Chapter*
> Appendix H: Rpmsg: Remote Processor Messaging
> \end_layout
>
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