[PATCH 01/14] content: explain that legacy support is optional

["Michael S. Tsirkin" <mst@redhat.com>]

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 content.tex | 31 +++++++++++++++++++++----------
 1 file changed, 21 insertions(+), 10 deletions(-)

diff --git a/content.tex b/content.tex
index f2f3132..9f01ab0 100644
--- a/content.tex
+++ b/content.tex
@@ -91,8 +91,9 @@ space.
 Earlier drafts of this specification (up to 0.9.X) defined a similar, but
 different interface between the hypervisor and the guest.
 Since these are widely deployed, this specification
-accommodates optional features to simplify transition
-from these earlier draft interfaces. Specifically:
+accommodates OPTIONAL features to simplify transition
+from these earlier draft interfaces. Specifically
+devices and drivers MAY support:
 
 \begin{description}
 \item[Legacy Interface]
@@ -110,33 +111,43 @@ Legacy devices and legacy drivers are not compliant with this
 specification.
 
 To simplify transition from these earlier draft interfaces,
-it is possible to implement:
+a device MAY implement:
 
 \begin{description}
 \item[Transitional Device]
         a device supporting both drivers conforming to this
         specification, and allowing legacy drivers.
+\end{description}
 
+Similarly, a driver MAY implement:
+\begin{description}
 \item[Transitional Driver]
         a driver supporting both devices conforming to this
         specification, and legacy devices.
 \end{description}
 
-Transitional devices and transitional drivers can be compliant with
-this specification (ie. when not operating in legacy mode).
-
 Devices or drivers with no legacy compatibility are referred to as
 non-transitional devices and drivers, respectively.
 
-Transitional Drivers can detect Legacy Devices by detecting that
+Transitional Drivers MUST detect Legacy Devices by detecting that
 the feature bit VIRTIO_F_VERSION_1 is not offered.
-Transitional devices can detect Legacy drivers by detecting that
+Transitional devices MUST detect Legacy drivers by detecting that
 VIRTIO_F_VERSION_1 has not been acknowledged by the driver.
 In this case device is used through the legacy interface.
 
+Legacy interface support is OPTIONAL.
+Thus, both transitional and non-transitional devices and
+drivers are compliant with this specification.
+
 To make them easier to locate, specification sections documenting
-these transitional features are explicitly marked with 'Legacy
-Interface' in the section title.
+features limited to transitional devices and transitional drivers
+are explicitly marked with 'Legacy Interface' in the section title.
+
+When device is used through the legacy interface, transitional
+devices and transitional drivers MUST operate according to the
+requirements documented within these legacy interface sections.
+Specification text within these sections generally does not apply
+to non-transitional devices.
 
 \section{Device Configuration Space}\label{sec:Basic Facilities of a Virtio Device / Device Configuration Space}
 
-- 
MST