net: Add passt network backend

This commit introduces support for passt as a new network backend.
passt is an unprivileged, user-mode networking solution that provides
connectivity for virtual machines by launching an external helper process.

The implementation reuses the generic stream data handling logic. It
launches the passt binary using GSubprocess, passing it a file
descriptor from a socketpair() for communication. QEMU connects to
the other end of the socket pair to establish the network data stream.

The PID of the passt daemon is tracked via a temporary file to
ensure it is terminated when QEMU exits.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

1 files changed, 38 insertions, 2 deletions

diff --git a/docs/system/devices/net.rst b/docs/system/devices/net.rst
index a3efbdc..c586ee0 100644
--- a/docs/system/devices/net.rst
+++ b/docs/system/devices/net.rst
@@ -85,13 +85,49 @@ passt doesn't require any capability or privilege. passt has
 better performance than ``-net user``, full IPv6 support and better security
 as it's a daemon that is not executed in QEMU context.
 
-passt can be connected to QEMU either by using a socket
-(``-netdev stream``) or using the vhost-user interface (``-netdev vhost-user``).
+passt_ can be used in the same way as the user backend (using ``-net passt``,
+``-netdev passt`` or ``-nic passt``) or it can be launched manually and
+connected to QEMU either by using a socket (``-netdev stream``) or by using
+the vhost-user interface (``-netdev vhost-user``).
+
+Using ``-netdev stream`` or ``-netdev vhost-user`` will allow the user to
+enable functionalities not available through the passt backend interface
+(like migration).
+
 See `passt(1)`_ for more details on passt.
 
 .. _passt: https://passt.top/
 .. _passt(1): https://passt.top/builds/latest/web/passt.1.html
 
+To use the passt backend interface
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+There is no need to start the daemon as QEMU will do it for you.
+
+passt is started in the socket-based mode.
+
+.. parsed-literal::
+   |qemu_system| [...OPTIONS...] -nic passt
+
+   (qemu) info network
+   e1000e.0: index=0,type=nic,model=e1000e,macaddr=52:54:00:12:34:56
+    \ #net071: index=0,type=passt,stream,connected to pid 24846
+
+.. parsed-literal::
+   |qemu_system| [...OPTIONS...] -net nic -net passt,tcp-ports=10001,udp-ports=10001
+
+   (qemu) info network
+   hub 0
+    \ hub0port1: #net136: index=0,type=passt,stream,connected to pid 25204
+    \ hub0port0: e1000e.0: index=0,type=nic,model=e1000e,macaddr=52:54:00:12:34:56
+
+.. parsed-literal::
+   |qemu_system| [...OPTIONS...] -netdev passt,id=netdev0 -device virtio-net,mac=9a:2b:2c:2d:2e:2f,id=virtio0,netdev=netdev0
+
+   (qemu) info network
+   virtio0: index=0,type=nic,model=virtio-net-pci,macaddr=9a:2b:2c:2d:2e:2f
+    \ netdev0: index=0,type=passt,stream,connected to pid 25428
+
 To use socket based passt interface:
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^