[PATCH v2] content: clarify feature negotiation terminology and init sequence

Make several clarifications to the init sequence documentation:

The Linux virtio core (drivers/virtio/virtio.c) initializes devices
as follows:
  1. Intersect driver and device feature bits
  2. finalize_features() - write accepted features to the device
  3. drv->validate() - read config space, may clear feature bits
     (e.g. virtio-net clears VIRTIO_NET_F_MTU if mtu < MIN_MTU,
     balloon clears PAGE_POISON if guest does not init pages)
  4. If validate changed any features, finalize_features() again
  5. virtio_features_ok() - set FEATURES_OK, confirm with device

this allows the device to know which fields will be read:
recommend this in the spec.

Legacy driver detection is specified using a mechanism that
does not work on all transports. Make it clear that it's an
example: what matters is that devices do detection in some way
and are compatible with legacy drivers.

Define "negotiated" for features confirmed via FEATURES_OK.

"acknowledged" is used as a synonym for "accepted", but only in two
places. Just use "accepted" consistently.

Spec describes multiple moving pieces then ends with "before accepting
it" - vague, and is overloading "accept". Replace with a reference to
FEATURES_OK.

Fixes: https://github.com/oasis-tcs/virtio-spec/issues/241
Signed-off-by: Michael S. Tsirkin <[email protected]>
---

changes:
address review comments by Stefan

 content.tex | 28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/content.tex b/content.tex
index 5de811f..0f4d662 100644
--- a/content.tex
+++ b/content.tex
@@ -39,7 +39,7 @@ \section{\field{Device Status} Field}\label{sec:Basic Facilities of a Virtio Dev
 \item[DRIVER_OK (4)] Indicates that the driver is set up and ready to
   drive the device.
 
-\item[FEATURES_OK (8)] Indicates that the driver has acknowledged all the
+\item[FEATURES_OK (8)] Indicates that the driver has accepted all the
   features it understands, and feature negotiation is complete.
 
 \item[SUSPEND (16)] When VIRTIO_F_SUSPEND is negotiated, indicates that the
@@ -89,13 +89,16 @@ \section{Feature Bits}\label{sec:Basic Facilities of a Virtio Device / Feature B
 
 Each virtio device offers all the features it understands.  During
 device initialization, the driver reads this and tells the device the
-subset that it accepts.  The only way to renegotiate is to reset
-the device.
+subset that it accepts.  The device validates this subset and
+either completes the negotiation successfully (the last subset of features
+that the driver accepted is considered negotiated then) or fails,
+leaving the feature negotiation incomplete. Once the negotiation is
+complete, the only way to renegotiate is to reset the device.
 
 This allows for forwards and backwards compatibility: if the device is
 enhanced with a new feature bit, older drivers will not write that
 feature bit back to the device.  Similarly, if a driver is enhanced with a feature
-that the device doesn't support, it see the new feature is not offered.
+that the device doesn't support, it will see that the new feature is not offered.
 
 Feature bits are allocated as follows:
 
@@ -189,8 +192,8 @@ \subsection{Legacy Interface: A Note on Feature
 
 Transitional Drivers MUST detect Legacy Devices by detecting that
 the feature bit VIRTIO_F_VERSION_1 is not offered.
-Transitional devices MUST detect Legacy drivers by detecting that
-VIRTIO_F_VERSION_1 has not been acknowledged by the driver.
+Transitional devices MUST detect Legacy drivers, e.g. by detecting that
+VIRTIO_F_VERSION_1 has not been accepted by the driver.
 
 In this case device is used through the legacy interface.
 
@@ -314,6 +317,11 @@ \section{Device Configuration Space}\label{sec:Basic Facilities of a Virtio Devi
 greater than the specified 8-bit size.
 \end{note}
 
+\drivernormative{\subsection}{Device Configuration Space}{Basic Facilities of a Virtio Device / Device Configuration Space}
+Before reading a device-specific configuration field that is
+conditional on a feature bit, the driver SHOULD first accept
+that feature bit.
+
 \devicenormative{\subsection}{Device Configuration Space}{Basic Facilities of a Virtio Device / Device Configuration Space}
 The device MUST allow reading of any device-specific configuration
 field before FEATURES_OK is set by the driver.  This includes fields which are
@@ -530,7 +538,13 @@ \section{Device Initialization}\label{sec:General Initialization And Device Oper
 \item\label{itm:General Initialization And Device Operation /
 Device Initialization / Read feature bits} Read device feature bits, and write the subset of feature bits
    understood by the OS and driver to the device.  During this step the
-   driver MAY read (but MUST NOT write) the device-specific configuration fields to check that it can support the device before accepting it.
+   driver MAY read (but MUST NOT write) the device-specific configuration
+   fields to check that it can support the device before setting FEATURES_OK.
+   The driver SHOULD accept feature bits before reading configuration
+   fields conditional on them.  The driver MAY then accept a different
+   subset of feature bits (e.g., deciding, based on the configuration
+   fields, not to use a certain feature), tell the device about the
+   updated subset, and repeat this process.
 
 \item\label{itm:General Initialization And Device Operation / Device Initialization / Set FEATURES-OK} Set the FEATURES_OK status bit.  The driver MUST NOT accept
    new feature bits after this step.
-- 
MST