virtio — archive

[Cornelia Huck <mst@redhat.com>]

On Wed, 1 Apr 2015 17:06:17 +0200 "Michael S. Tsirkin" <mst@redhat.com> wrote: > On Wed, Apr 01, 2015 at 04:44:41PM +0200, Cornelia Huck wrote: > > 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? > > > > (...) > > Merely because we shouldn't have MUST keywords outside > normative statements. Ah, that makes sense. > Maybe I should make this a separate patch, and queue for 1.0. > > > > > @@ -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? > > How about a slightly more specific "despite the usual > convention that legacy device fields are guest endian"? Sounds good.