On Mon, Mar 06, 2023 at 01:53:50PM +0000, David Edmondson wrote:
> "Michael S. Tsirkin" <> 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 <>
>
> Signed-off-by: Michael S. Tsirkin <>
> > ---
>
> 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?
>
> 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.
>
> "require changed 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 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.
> > +
> > \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 features the driver accepts, otherwise it MUST fail to set the
>
> FEATURES_OK \field{device status} bit when the driver writes it.
> >
> > +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
> >
> >
> >
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