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Subject: Re: [virtio] [PATCH v10 01/10] virtio: document forward compatibility guarantees
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..." ? I'm not sure how we can make this both short and descriptive enough...
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