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 <> Reviewed-by: Parav Pandit <> --- Changes in v11: make explanation about not using any unspecified bits more detailed as suggested by Cornelia and David Reviewed-by: Zhu Lingshan <> --- content.tex
44 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 44 insertions(+) diff --git a/content.tex b/content.tex index cff548a..4ec2d76 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 +egin{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
