Re: [PATCH v3 4/4] docs: vhost-user: add subsection for non-Linux platforms

[Stefan Hajnoczi]

On Thu, Mar 03, 2022 at 12:59:11PM +0100, Sergio Lopez wrote:
> Add a section explaining how vhost-user is supported on platforms
> other than Linux.
> 
> Signed-off-by: Sergio Lopez <slp@redhat.com>
> ---
>  docs/interop/vhost-user.rst | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
> index edc3ad84a3..590a626b92 100644
> --- a/docs/interop/vhost-user.rst
> +++ b/docs/interop/vhost-user.rst
> @@ -38,6 +38,24 @@ conventions <backend_conventions>`.
>  *Master* and *slave* can be either a client (i.e. connecting) or
>  server (listening) in the socket communication.
>  
> +Support for platforms other than Linux
> +--------------------------------------
> +
> +While vhost-user was initially developed targeting Linux, nowadays is

s/is/it is/

> +supported on any platform that provides the following features:
> +
> +- The ability to share a mapping injected into the guest between
> +  multiple processes, so both QEMU and the vhost-user daemon servicing
> +  the device can access simultaneously the memory regions containing
> +  the virtqueues and the data associated with each request.

Please generalize this statement since there are other vhost-user
protocol features aside from guest RAM access that involve shared
memory:
1. VHOST_USER_SET_LOG_BASE
2. VHOST_USER_SET_INFLIGHT_FD

The exact requirement is:

  The vhost-user protocol relies on shared memory represented by a file
  descriptor so it can be passed over a UNIX domain socket and then
  mapped by the other process.

> +
> +- AF_UNIX sockets with SCM_RIGHTS, so QEMU can communicate with the
> +  vhost-user daemon and send it file descriptors when needed.
> +
> +- Either eventfd or pipe/pipe2. On platforms where eventfd is not
> +  available, QEMU will automatically fallback to pipe2 or, as a last

The noun is "fallback", the verb form is "fall back":
s/fallback/fall back/

It's worth mentioning that events are sent over pipe fds by writing an
8-byte value. The 8-byte value has no meaning and should not be
interpreted.

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