>
From: Michael S. Tsirkin <>
> Sent: Thursday, February 9, 2023 7:14 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 <>
> 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.) 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
> 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
Reviewed-by: Parav Pandit <>
