virtio — archive

[PATCH v11 00/10] Introduce device group and device management

From: Lingshan Zhu <[email protected]>
To: [email protected]
Date: Thu, 06 Apr 2023 07:16:00 -0000

On 4/3/2023 11:03 PM, Michael S. Tsirkin wrote: > Feature negotiation forms the basis of forward compatibility > guarantees of virtio but has never been properly documented. > Do it now. > > Suggested-by: Halil Pasic <[email protected]> > Signed-off-by: Michael S. Tsirkin <[email protected]> > Reviewed-by: Parav Pandit <[email protected]> > --- > > Changes in v11: > make explanation about not using any unspecified bits > more detailed as suggested by Cornelia and David > --- > content.tex | 44 ++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 44 insertions(+) > > diff --git a/content.tex b/content.tex > index 0e474dd..8098988 100644 > --- a/content.tex > +++ b/content.tex > @@ -114,21 +114,65 @@ \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 any unspecified, > +reserved, or unsupported features even if offered, according to > +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 any unspecified, > +reserved, or unsupported features in the first place, > +according to 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 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 Reviewed-by: Zhu Lingshan <[email protected]>