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

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

On Thu, Mar 02, 2023 at 06:39:29PM +0000, Parav Pandit wrote:
> 
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Thursday, March 2, 2023 8:05 AM
> > 
> > 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>
> > ---
> 
> It may be little painful, but in 10 patch series, it is worth having per patch change log.
> Because I do not know my reviewed by tag in [1] was dropped because of a big change, if so what was it or it was simply missed out.
> 
> [1] https://lists.oasis-open.org/archives/virtio-dev/202302/msg00237.html

I just lost it, no change.

> >  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.) Likewise, it is important that
> > +drivers \em{do not} accept feature bits they do not know how to handle
> > +(even though devices are not supposed to offer them in the first place,
> > +according to this version of the specification.) The preferred way for
> > +handling reserved and unexpected features is that the driver ignores
> > +them.
> > +
> > +In particular, this is
> > +especially important for features limited to specific transports, as
> > +enabling these for more transports in future versions of the
> > +specification is highly likely to require changing the behaviour from
> > +drivers and devices.  Drivers and devices supporting multiple
> > +transports need to carefully maintain per-transport lists of allowed
> > +features.
> > +
> >  \drivernormative{\subsection}{Feature Bits}{Basic Facilities of a Virtio Device /
> > Feature Bits}  The driver MUST NOT accept a feature which the device did not
> > offer,  and MUST NOT accept a feature which requires another feature which
> > was  not accepted.
> > 
> > +The driver MUST validate the feature bits offered by the device.
> > +The driver MUST ignore and MUST NOT accept any feature bit that is
> > +\begin{itemize} \item not described in this specification, \item marked
> > +as reserved, \item not supported for the specific transport, \item not
> > +defined for the device type.
> > +\end{itemize}
> > +
> >  The driver SHOULD go into backwards compatibility mode  if the device does
> The grammar tool is suggesting s/backwards/backward.
> 
> > not offer a feature it understands, otherwise MUST  set the FAILED \field{device
> > status} bit and cease initialization.
> > 
> > +By contrast, the driver MUST NOT fail solely because a feature it does
> > +not understand has been offered by the device.
> > +
> Above line can be written without introducing "has been" given we have only two entities (driver, dev) to care about.
> 
> By contrast, the driver MUST NOT fail solely because it does not understand the feature offered by the device.
> 
> >  \devicenormative{\subsection}{Feature Bits}{Basic Facilities of a Virtio Device /
> > Feature Bits}  The device MUST NOT offer a feature which requires another
> > feature  which was not offered.  The device SHOULD accept any valid subset  of
> Extra white space between feature and which, subset and of.
> 
> > features the driver accepts, otherwise it MUST fail to set the  FEATURES_OK
> > \field{device status} bit when the driver writes it.
> > 
> Extra white space between the FEATURE_OK
> 
> > +The device MUST NOT offer feature bits corresponding to features it
> > +would not support if accepted by the driver (even if the driver is
> > +prohibited from accepting the feature bits by the specification); for
> > +the sake of clarity, this refers to 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, but
> > +this does not preclude devices written to a future version of this
> > +specification from offering such feature bits should such a
> > +specification have a provision for devices to support the corresponding
> > +features.
> > +
> >  If a device has successfully negotiated a set of features  at least once (by
> > accepting the FEATURES_OK \field{device  status} bit during device
> > initialization), then it SHOULD
> > --
> > MST