Re: [PATCH v2 3/7] docs: document non-net VHOST_USER_GET_FEATURES behavior

[Stefan Hajnoczi]

On Wed, Jun 10, 2020 at 01:01:26AM -0400, Michael S. Tsirkin wrote:
> On Tue, Jun 09, 2020 at 06:02:14PM +0100, Stefan Hajnoczi wrote:
> > +For devices other than the networking device, masters may assume the 
> > following
> > +feature bits are always set in ``VHOST_USER_GET_FEATURES`` for 
> > compatibility
> > +with legacy backend implementations that do not report them correctly:
> > +* ``VIRTIO_F_RING_INDIRECT_DESC``
> > +* ``VIRTIO_F_RING_EVENT_IDX``
> > +* ``VIRTIO_F_VERSION_1``
> > +* ``VIRTIO_F_NOTIFY_ON_EMPTY``
> > +* ``VIRTIO_F_ANY_LAYOUT``
> > +
> >  Starting and stopping rings
> >  ---------------------------
> 
> How common are these backends? Anything shipped for a while?  IIUC we
> are not talking about years of history here, so I really think we should
> just enforce what spec always said, rather than work around some broken
> clients.

QEMU didn't include some device backends in feature negotiation. This is
why we didn't notice that libvhost-user does not advertise some feature
bits. Therefore the chance of third-party device backends missing
feature negotiation seems high too.

I will move this section into the VHOST_USER_GET_FEATURES description.

Stefan

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