From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:50760) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bty1G-0007rB-Gs for qemu-devel@nongnu.org; Tue, 11 Oct 2016 10:27:03 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bty1C-0007rl-9t for qemu-devel@nongnu.org; Tue, 11 Oct 2016 10:27:01 -0400 Received: from mx1.redhat.com ([209.132.183.28]:49046) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bty1C-0007rU-3m for qemu-devel@nongnu.org; Tue, 11 Oct 2016 10:26:58 -0400 Date: Tue, 11 Oct 2016 15:26:54 +0100 From: "Daniel P. Berrange" Message-ID: <20161011142654.GJ14917@redhat.com> Reply-To: "Daniel P. Berrange" References: <20161011102637.GA28347@zurbaran.home> <20161011131107.GF14917@redhat.com> <20161011140834.GA16796@work-vm> <20161011141704.GI14917@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20161011141704.GI14917@redhat.com> Subject: Re: [Qemu-devel] Multiple QMP socket clients List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Dr. David Alan Gilbert" Cc: Samuel Ortiz , qemu-devel@nongnu.org On Tue, Oct 11, 2016 at 03:17:04PM +0100, Daniel P. Berrange wrote: > On Tue, Oct 11, 2016 at 03:08:34PM +0100, Dr. David Alan Gilbert wrote: > > * Daniel P. Berrange (berrange@redhat.com) wrote: > > > On Tue, Oct 11, 2016 at 12:26:37PM +0200, Samuel Ortiz wrote: > > > > Hi, > > > > > > > > On a qemu instance started with a qmp unix socket: > > > > > > > > -qmp unix:/tmp/ctrl.sock,server,nowait > > > > > > > > I am trying to have multiple clients working on that socket but > > > > although the qmp server seems to accept the connections, only the first > > > > connected client gets his request processed. The next client requests > > > > will just hang until the first one exits. > > > > > > > > Is that an intended behaviour ? > > > > > > Yes, the character device code is designed around the idea of a single > > > endpoint. > > > > > > In the case of the monitor you could work around it by adding multiple > > > -qmp arguemnts, each with different socket. Of course you have to make > > > sure each client doesn't trample on the other client when doing this. > > > > But why does it accept the connection? > > I thought you could say only accept a single connection on a socket. > > (The backlog parameter to listen(2) but I can't find out listen.) > > QEMU won't accept the connection, as while it is still listen()ing on > the socket, it is not poll()ing for incoming clients, so will never > trigger accept(). The kernel will queue the incoming connection until > QEMU starts polling for clients again. From the client POV this is > indistinguishable from QEMU accepting the client, but not processing > I/O on it. Oh, and the backlog parameter to listen() is fairly useless - it won't stop new clients getting into an established socket state https://veithen.github.io/2014/01/01/how-tcp-backlog-works-in-linux.html QEMU could just accept() all incoming connections and explicitly close them if something is already connected, but this just burns CPU really. Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://entangle-photo.org -o- http://search.cpan.org/~danberr/ :|