Re: [PATCH v3 4/4] docs: Add documentation for vhost based RNG implementation

[Alex Bennée]

Mathieu Poirier <mathieu.poirier@linaro.org> writes:

> 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/index.rst          |  1 +
>  docs/tools/vhost-user-rng.rst | 74 +++++++++++++++++++++++++++++++++++
>  2 files changed, 75 insertions(+)
>  create mode 100644 docs/tools/vhost-user-rng.rst
>
> diff --git a/docs/tools/index.rst b/docs/tools/index.rst
> index d923834a7398..9d80fa89eceb 100644
> --- a/docs/tools/index.rst
> +++ b/docs/tools/index.rst
> @@ -15,5 +15,6 @@ Contents:
>     qemu-nbd
>     qemu-pr-helper
>     qemu-trace-stap
> +   vhost-user-rng
>     virtfs-proxy-helper
>     virtiofsd
> diff --git a/docs/tools/vhost-user-rng.rst b/docs/tools/vhost-user-rng.rst
> new file mode 100644
> index 000000000000..7f69d7bb3c58
> --- /dev/null
> +++ 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                                                  
> \
> +      ...

Would it be worth pointing out how a guest may consume the randomness? I
appreciate this will be guest specific but currently I'm struggling how
to consume the entropy in a Linux guest.

  cat /dev/urandom > /dev/null

didn't seem to cause any to be consumed above what was during boot up.

-- 
Alex Bennée