From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:39578) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cZIk6-0004Lj-5f for qemu-devel@nongnu.org; Thu, 02 Feb 2017 09:52:11 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cZIk4-00045U-Hj for qemu-devel@nongnu.org; Thu, 02 Feb 2017 09:52:10 -0500 Received: from mx1.redhat.com ([209.132.183.28]:44152) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cZIk4-000447-C7 for qemu-devel@nongnu.org; Thu, 02 Feb 2017 09:52:08 -0500 From: "Richard W.M. Jones" Date: Thu, 2 Feb 2017 14:51:58 +0000 Message-Id: <20170202145159.20440-1-rjones@redhat.com> Subject: [Qemu-devel] [PATCH] qemu-nbd: Implement socket activation. List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: pbonzini@redhat.com Cc: qemu-devel@nongnu.org, den@openvz.org, rkagan@virtuozzo.com, dplotnikov@virtuozzo.com, berrange@redhat.com, stefanha@gmail.com Socket activation (sometimes known as systemd socket activation) allows an Internet superserver to pass a pre-opened listening socket to the process, instead of having qemu-nbd open a socket itself. This is done via the LISTEN_FDS and LISTEN_PID environment variables, and a standard file descriptor range. This patch partially implements socket activation. The limitation of this implementation is that qemu-nbd can only listen on a single file descriptor, and so if LISTEN_FDS > 1 (eg. for listening on multiple interfaces or ports) socket activation will fail. However for the simple case of listening on a single port, and either all interfaces with IPv4+IPv6, or just a loopback interface, the current implementation works fine. Fixing this properly would require considerable changes throughout qemu, since qemu's currently handling of getaddrinfo is plainly wrong. To use qemu-nbd from systemd, you create /etc/systemd/system/nbd.socket: [Unit] Description=QEMU Network Block Device server [Socket] ListenStream=10809 [Install] WantedBy=sockets.target and /etc/systemd/system/nbd.service: [Service] ExecStart=/usr/sbin/qemu-nbd -v -t /path/to/file and enable the socket service (only): systemctl enable nbd.socket systemctl start nbd.socket and then connecting to port 10809 will start qemu-nbd and service the file, with systemd opening the listening socket. In the ExecStart line, the qemu-nbd -v option is only needed if you want enhanced debugging. The -t option is required unless you want to fiddle with systemd settings for rate-limiting. If you try to use the -p and similar options with socket activation then qemu-nbd will give an error. (I wasn't sure where to document this -- there is no obvious documentation for qemu-nbd beyond the simple list of command line arguments) This is based on the implementations in libvirt (src/util/virutil.c:virGetListenFDs) and nbdkit (src/main.c:get_socket_activation), and also on Denis Plotnikov's implementation of --server-sock-fd (https://lists.gnu.org/archive/html/qemu-devel/2016-09/msg07781.html). Rich.