[PATCH v2] docs: Update vhost-user spec regarding backend program conventions

[Sebastien Boeuf <sebastien.boeuf@intel.com>]

The vhost-user specification is not clearly stating the expected
behavior from a backend program whenever the client disconnects.

This patch addresses the issue by defining the default behavior and
proposing an alternative through a command line option.

By default, a backend program will terminate whenever the client
disconnects, unless told otherwise through the newly introduced
option --keep-listening.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 docs/interop/vhost-user.rst | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
index 5f8b3a456b..078f8e2e57 100644
--- a/docs/interop/vhost-user.rst
+++ b/docs/interop/vhost-user.rst
@@ -1323,6 +1323,10 @@ The backend program must end (as quickly and cleanly as possible) when
 the SIGTERM signal is received. Eventually, it may receive SIGKILL by
 the management layer after a few seconds.
 
+By default, the backend program terminates after the client disconnects.
+This is useful to keep the backend program's lifetime synchronized with
+its client process.
+
 The following command line options have an expected behaviour. They
 are mandatory, unless explicitly said differently:
 
@@ -1337,6 +1341,14 @@ are mandatory, unless explicitly said differently:
   vhost-user socket as file descriptor FDNUM. It is incompatible with
   --socket-path.
 
+--keep-listening
+
+  When this option is provided, the backend program must keep listening
+  after the client disconnects. It accepts only 1 connection at a time
+  on each UNIX domain socket. This prevents the management layer from
+  having to spawn a new backend program each and every time the client
+  disconnects.
+
 --print-capabilities
 
   Output to stdout the backend capabilities in JSON format, and then
-- 
2.20.1

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