Re: [virtio] [PATCH v10 01/10] virtio: document forward compatibility guarantees

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Mar 08, 2023 at 03:16:41PM +0100, Cornelia Huck wrote:
> On Tue, Mar 07 2023, David Edmondson <david.edmondson@oracle.com> wrote:
> 
> > "Michael S. Tsirkin" <mst@redhat.com> writes:
> >
> >> On Mon, Mar 06, 2023 at 01:53:50PM +0000, David Edmondson wrote:
> >>> "Michael S. Tsirkin" <mst@redhat.com> writes:
> >>> 
> >>> > Feature negotiation forms the basis of forward compatibility
> >>> > guarantees of virtio but has never been properly documented.
> >>> > Do it now.
> >>> >
> >>> > Suggested-by: Halil Pasic <pasic@linux.ibm.com>
> >>> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> >>> > ---
> >>> >  content.tex | 42 ++++++++++++++++++++++++++++++++++++++++++
> >>> >  1 file changed, 42 insertions(+)
> >>> >
> >>> > diff --git a/content.tex b/content.tex
> >>> > index 0e474dd..0c2d917 100644
> >>> > --- a/content.tex
> >>> > +++ b/content.tex
> >>> > @@ -114,21 +114,63 @@ \section{Feature Bits}\label{sec:Basic Facilities of a Virtio Device / Feature B
> >>> >  In particular, new fields in the device configuration space are
> >>> >  indicated by offering a new feature bit.
> >>> >  
> >>> > +To keep the feature negotiation mechanism extensible, it is important
> >>> > +that devices \em{do not} offer any feature bits that they would not be
> >>> > +able to handle if the driver accepted them (even though drivers are not
> >>> > +supposed to accept them in the first place even if offered, according to
> >>> > +this version of the specification.) 
> >>> 
> >>> I find this (the bit in parenthesis) confusing.
> >>> 
> >>> Why are drivers not supposed to accept features that they have been
> >>> offered, given that they can't know that the device cannot handle the
> >>> feature that it just offered?
> >>> 
> >>> Is this alluding to the later section:
> >>> 
> >>> > feature bits not described in this specification, reserved feature
> >>> > bits and feature bits reserved or not supported for the specific
> >>> > transport or the specific device type
> >>> 
> >>> ?
> >>
> >> exactly. how would you put this better? given an example?
> >
> > Perhaps it would be enough to say:
> >
> >> (even though drivers are not supposed to accept unrecognised features in
> >> the first place even if offered, according to the specification)
> >
> > "Unrecognised" is intended as a shorthand for the whole "not described,
> > reserved, ...". Maybe "unrecognised or reserved"?
> 
> Hm, what about
> 
> "even though drivers are not supposed to accept any unspecified,
> reserved, or unsupported features even if offered..."
> 
> ?

ok

> I'm not sure how we can make this both short and descriptive enough...