[virtio-comment] [PATCH v4 1/3] shared memory: Define shared memory regions

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

Define the requirements and idea behind shared memory regions.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 conformance.tex |  1 +
 content.tex     |  2 ++
 shared-mem.tex  | 40 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 43 insertions(+)
 create mode 100644 shared-mem.tex

diff --git a/conformance.tex b/conformance.tex
index ad7e82e..f6c0d50 100644
--- a/conformance.tex
+++ b/conformance.tex
@@ -195,6 +195,7 @@ A device MUST conform to the following normative statements:
 \item \ref{devicenormative:Basic Facilities of a Virtio Device / Virtqueues / Used Buffer Notification Suppression}
 \item \ref{devicenormative:Basic Facilities of a Virtio Device / Virtqueues / The Virtqueue Used Ring}
 \item \ref{devicenormative:Basic Facilities of a Virtio Device / Virtqueues / Available Buffer Notification Suppression}
+\item \ref{devicenormative:Basic Facilities of a Virtio Device / Shared Memory Regions}
 \item \ref{devicenormative:Reserved Feature Bits}
 \end{itemize}
 
diff --git a/content.tex b/content.tex
index ede0ef6..8cd1a38 100644
--- a/content.tex
+++ b/content.tex
@@ -371,6 +371,8 @@ making any more buffers available. When VIRTIO_F_NOTIFICATION_DATA
 has been negotiated, these notifications would then have
 identical \field{next_off} and \field{next_wrap} values.
 
+\input{shared-mem.tex}
+
 \chapter{General Initialization And Device Operation}\label{sec:General Initialization And Device Operation}
 
 We start with an overview of device initialization, then expand on the
diff --git a/shared-mem.tex b/shared-mem.tex
new file mode 100644
index 0000000..6dc506a
--- /dev/null
+++ b/shared-mem.tex
@@ -0,0 +1,40 @@
+\section{Shared Memory Regions}\label{sec:Basic Facilities of a Virtio Device / Shared Memory Regions}
+
+Shared memory regions are an additional facility
+available to devices that need a region of memory that's
+continuously shared between the host and the guest, rather
+than passed between them in the way virtqueue elements are.
+
+Example uses include shared caches and version pools for versioned
+data structures.
+
+The region is chosen by the host and presented to the guest, as
+such it is useful in situations where the memory is accessed on
+the host by other libraries that can't safely access guest RAM.
+
+A device may have multiple shared memory regions associated with
+it.  Each region has a \field{shmid} to identify it, the meaning
+of which is device-specific.
+
+Enumeration and location of shared memory regions is performed
+using a transport-specific data structure and mechanism.
+
+Memory consistency rules vary depending on the region and the
+device and they will be specified as required by each device.
+
+\subsection{Addressing within regions}\label{sec:Basic Facilities of a Virtio Device / Shared Memory Regions / Addressing within regions }
+
+References into shared memory regions are represented as offsets from
+the beginning of the region instead of absolute memory addresses.
+Offsets are used both for references between structures stored
+within shared memory and for requests placed in virtqueues that
+refer to shared memory.
+The \field{shmid} may be explicit or may be inferred from the
+context of the reference.
+
+\devicenormative{\subsection}{Shared Memory Regions}{Basic Facilities of a Virtio
+Device / Shared Memory Regions}
+Shared memory regions MUST NOT expose shared memory regions which
+are used to control the operation of the device, nor to stream
+data.
+
-- 
2.20.1


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