[PATCH 4/4] vhost-user: update shared memory capability

The virtio vhost-user device has shared memory resources. Therefore, it
uses the VIRTIO_PCI_CAP_SHARED_MEMORY_CFG capability to standardize
those resources through the PCI Configuration Space. This patch
synchronizes the virtio vhost-user device spec with the following shared
memory resources PATCH:

https://lists.oasis-open.org/archives/virtio-dev/201902/msg00142.html

Signed-off-by: Nikos Dragazis <>
---
 virtio-vhost-user.tex | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/virtio-vhost-user.tex b/virtio-vhost-user.tex
index 605272f..4a2fee3 100644
--- a/virtio-vhost-user.tex
+++ b/virtio-vhost-user.tex
@@ -318,7 +318,22 @@ \subsubsection{Notification structure layout}\label{sec:Device Types / Vhost-use
 \subsubsection{Shared memory capability}\label{sec:Device Types / Vhost-user Device Backend / Additional Device Resources over PCI / Shared Memory capability}
 
 The shared memory location is found using the VIRTIO_PCI_CAP_SHARED_MEMORY_CFG
-capability.
+capability. Using the additional \field{offset_hi} and \field{length_hi}
+fields following the SHARED_MEMORY_CFG capability structure, the shared
+memory region's 64-bit BAR offset is calculated as follows:
+
+\begin{lstlisting}
+        offset_hi << 32 + cap.offset
+\end{lstlisting}
+
+and the shared memory region's 64-bit length is calculated as follows:
+
+\begin{lstlisting}
+        length_hi << 32 + cap.length
+\end{lstlisting}
+
+The \field{cap.offset} and \field{cap.length} fields are taken from the
+SHARED_MEMORY_CFG capability structure.
 
 \devicenormative{\paragraph}{Shared Memory capability}{Device Types / Vhost-user Device Backend / Additional Device Resources over PCI / Shared Memory capability}
 The device MUST present exactly one shared memory capability.
-- 
2.7.4