[virtio-dev] [PATCH v13 01/10] virtio: document forward compatibility guarantees

Michael S. Tsirkin
2023-05-05T15:41:13Z

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>
Reviewed-by: Parav Pandit <parav@nvidia.com>
---

Changes in v11:
make explanation about not using any unspecified bits
more detailed as suggested by Cornelia and David
Reviewed-by: Zhu Lingshan <lingshan.zhu@intel.com>
---
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
+\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