creds: import 'vmm.notify_socket' and use it to set

This is intended to be used with VSOCK, to notify the hypervisor/VMM, eg on the host:

qemu <...> -smbios type=11,value=io.systemd.credential:vmm.notify_socket=vsock:2:1234 -device vhost-vsock-pci,id=vhost-vsock-pci0,guest-cid=42

(vsock:2:1234 -> send to host on vsock port 1234, default is to send to 0 which is
the hypervisor itself)

Also on the host:

$ socat - VSOCK-LISTEN:1234,socktype=5
READY=1
STATUS=Ready.

1 files changed, 18 insertions, 0 deletions

diff --git a/man/systemd.system-credentials.xml b/man/systemd.system-credentials.xml
index 3eadf9b985..2a87139a11 100644
--- a/man/systemd.system-credentials.xml
+++ b/man/systemd.system-credentials.xml
@@ -190,6 +190,24 @@
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>vmm.notify_socket</varname></term>
+        <listitem>
+          <para>This credential is parsed looking for an <constant>AF_VSOCK</constant> or
+          <constant>AF_UNIX</constant> address where to send a <constant>READY=1</constant>
+          notification datagram when the system has finished booting. See:
+          <citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+          This is useful for hypervisors/VMMs or other processes on the host
+          to receive a notification via VSOCK when a virtual machine has finished booting.
+          Note that in case the hypervisor does not support <constant>SOCK_DGRAM</constant>
+          over <constant>AF_VSOCK</constant>, <constant>SOCK_SEQPACKET</constant> will be
+          tried instead. The credential payload for <constant>AF_VSOCK</constant> should be
+          in the form: <literal>vsock:CID:PORT</literal>, where <literal>CID</literal> is
+          optional and if omitted will default to talking to the hypervisor
+          (<constant>0</constant>).</para>
+        </listitem>
+      </varlistentry>
+
     </variablelist>
   </refsect1>