[PATCH 4/5] docs: Add documentation for vhost based RNG implementation

[Mathieu Poirier]

Add description and example for the vhost-user based RNG implementation.
Tailored on Viresh Kumar's vhost-user-i2c documentation.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
---
 docs/tools/vhost-user-rng.rst | 74 +++++++++++++++++++++++++++++++++++
 1 file changed, 74 insertions(+)
 create mode 100644 docs/tools/vhost-user-rng.rst

diff --git a/docs/tools/vhost-user-rng.rst b/docs/tools/vhost-user-rng.rst
new file mode 100644
index 000000000000..7f69d7bb3c58
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+++ b/docs/tools/vhost-user-rng.rst
@@ -0,0 +1,74 @@
+QEMU vhost-user-rng - RNG emulation backend
+===========================================
+
+Synopsis
+--------
+
+**vhost-user-rng** [*OPTIONS*]
+
+Description
+-----------
+
+This program is a vhost-user backend that emulates a VirtIO random number
+generator (RNG).  It uses the host's random number generator pool,
+/dev/urandom by default but configurable at will, to satisfy requests from
+guests.
+
+This program is designed to work with QEMU's ``-device
+vhost-user-rng-pci`` but should work with any virtual machine monitor
+(VMM) that supports vhost-user. See the Examples section below.
+
+Options
+-------
+
+.. program:: vhost-user-rng
+
+.. option:: -h, --help
+
+  Print help.
+
+.. option:: -v, --verbose
+
+   Increase verbosity of output
+
+.. option:: -s, --socket-path=PATH
+
+  Listen on vhost-user UNIX domain socket at PATH. Incompatible with --fd.
+
+.. option:: -f, --fd=FDNUM
+
+  Accept connections from vhost-user UNIX domain socket file descriptor FDNUM.
+  The file descriptor must already be listening for connections.
+  Incompatible with --socket-path.
+
+.. option:: -p, --period
+
+  Rate, in milliseconds, at which the RNG hardware can generate random data.
+  Used in conjunction with the --max-bytes option.
+
+.. option:: -m, --max-bytes
+
+  In conjuction with the --period parameter, provides the maximum number of 
byte
+  per milliseconds a RNG device can generate.
+
+Examples
+--------
+
+The daemon should be started first:
+
+::
+
+  host# vhost-user-rng --socket-path=rng.sock --period=1000 --max-bytes=4096
+
+The QEMU invocation needs to create a chardev socket the device can
+use to communicate as well as share the guests memory over a memfd.
+
+::
+
+  host# qemu-system                                                            
\
+      -chardev socket,path=$(PATH)/rng.sock,id=rng0                            
\
+      -device vhost-user-rng-pci,chardev=rng0                                  
\
+      -m 4096                                                                  
\
+      -object memory-backend-file,id=mem,size=4G,mem-path=/dev/shm,share=on    
\
+      -numa node,memdev=mem                                                    
\
+      ...
-- 
2.25.1