Re: [PATCH virtio-spec] balloon: trannsitional device support

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Wed, 1 Apr 2015 14:59:46 +0200
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> Virtio 1.0 doesn't include a modern balloon device.  At some point we'll likely
> define an incompatible interface with a different ID and different
> semantics.  But for now, it's not a big effort to support a transitional
> balloon device: this has the advantage of supporting existing drivers,
> transparently, as well as transports that don't allow mixing virtio 0 and
> virtio 1 devices. And balloon is an easy device to test, so it's also useful
> for people to test virtio core handling of transitional devices.
> 
> The only interface issue is with the stats buffer, which has misaligned
> fields. We could leave it as is, but this sets a bad precedent that
> others might copy by mistake.
> 
> As we need to change stats code to do byteswaps for virtio 1.0, it seems easy
> to fix by prepending a 6 byte reserved field.  I also had to change config
> structure field types from __le32 to __u32 to match other devices. This means
> we need a couple of __force tags for legacy path but that seems minor.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  content.tex | 73 +++++++++++++++++++++++++++++++++++++++++++++++--------------
>  1 file changed, 56 insertions(+), 17 deletions(-)
> 

> @@ -4329,13 +4328,33 @@ guest memory statistics to the host.
> 
>  \subsection{Feature bits}\label{sec:Device Types / Memory Balloon Device / Feature bits}
>  \begin{description}
> -\item[VIRTIO_BALLOON_F_MUST_TELL_HOST (0)] Host MUST be told before
> -    pages from the balloon are used.
> +\item[VIRTIO_BALLOON_F_MUST_TELL_HOST (0)] Host has to be told before
> +    pages from the balloon are used by guest.

Why this change? The device only needs to tolerate the guest using
pages if that bit has not been negotiated?

(...)

> @@ -4347,24 +4366,29 @@ struct virtio_balloon_config {
>  };
>  \end{lstlisting}
> 
> -Note that these fields are always little endian, despite convention
> -that legacy device fields are guest endian.
> +\subparagraph{Legacy Interface: Device configuration layout}\label{sec:Device Types / Memory Balloon Device / Device
> +configuration layout / Legacy Interface: Device configuration layout}
> +When using the legacy interface, transitional devices and drivers
> +MUST format the fields in struct virtio_balloon_config
> +according to the little-endian format.

I'd like to keep it obvious that this is different from other legacy
devices, so let's append "...unlike the usual convention for legacy
devices"? Or do this in a footnote?

I think you also need to add the new normative sections to
conformance.tex.