From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5588BC2BA2B for ; Thu, 9 Apr 2020 13:26:12 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 22D2D2072F for ; Thu, 9 Apr 2020 13:26:12 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="dkWOx+F6" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 22D2D2072F Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:49216 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jMXC7-00066Y-BT for qemu-devel@archiver.kernel.org; Thu, 09 Apr 2020 09:26:11 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:57788) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jMXB5-0005Hj-9T for qemu-devel@nongnu.org; Thu, 09 Apr 2020 09:25:08 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1jMXAx-0002gp-5l for qemu-devel@nongnu.org; Thu, 09 Apr 2020 09:25:06 -0400 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:52138 helo=us-smtp-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1jMXAx-0002gb-0a for qemu-devel@nongnu.org; Thu, 09 Apr 2020 09:24:59 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1586438697; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=fSMAAS2Kq/qARUHD6JM5i3/dISvcdn1d1/wkkApc+JE=; b=dkWOx+F6eTspbFI18yveKtBEhDpMaz3ICxnzRxosOTGN1sg0OlG/X5TlTstb2Q4ndg2885 C957OBm+SxBsM5m/6wceefrhSeEpv4lcXnHb/G+fpSar4jFlysEov4WyPqOGTCCDe7ocC/ C0/QNIn5cSOcqA0b8LM4pAqi0vt6rFo= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-479-zkYDyK2gP-uUgkqAhdfkkA-1; Thu, 09 Apr 2020 09:24:54 -0400 X-MC-Unique: zkYDyK2gP-uUgkqAhdfkkA-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 29F23190B2A1; Thu, 9 Apr 2020 13:24:53 +0000 (UTC) Received: from redhat.com (unknown [10.36.110.6]) by smtp.corp.redhat.com (Postfix) with ESMTPS id D49A999E01; Thu, 9 Apr 2020 13:24:45 +0000 (UTC) Date: Thu, 9 Apr 2020 14:24:41 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: =?utf-8?Q?Marc-Andr=C3=A9?= Lureau Subject: Re: Replace GSource with AioContext for chardev Message-ID: <20200409132441.GS1202384@redhat.com> References: <20200409124601.toh6jpbfcwiwzb6z@r> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/1.13.3 (2020-01-12) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.120 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Fam Zheng , Kevin Wolf , qemu-devel , Coiby Xu , Stefan Hajnoczi , Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Apr 09, 2020 at 03:16:01PM +0200, Marc-Andr=C3=A9 Lureau wrote: > Hi >=20 > On Thu, Apr 9, 2020 at 2:46 PM Coiby Xu wrote: > > > > > > Hi, > > > > I'm now implementing vhost-user block device backend > > https://patchew.org/QEMU/20200309100342.14921-1-coiby.xu@gmail.com/ > > and want to use chardev to help manage vhost-user client connections > > and read socket message. However there are two issues that need to be > > addressed. > > > > Firstly, chardev isn't suitable for the case when exported drive is > > run in an IOThread because for mow chardev use GSource to dispatch > > socket fd events. So I have to specify which IOThread the exported > > drive is using when launching vhost-user block device backend, > > for example, the following syntax will be used, > > > > -drive file=3Dfile.img,id=3Ddisk -device virtio-blk,drive=3Ddisk,iot= hread=3Diothread0 \ > > -object vhost-user-blk-server,node-name=3Ddisk,chardev=3Dmon1,iothr= ead=3Diothread0 \ > > -object iothread,id=3Diothread0 \ > > -chardev socket,id=3Dmon1,path=3D/tmp/vhost-user-blk_vhost.socket,s= erver,nowait > > > > then iothread_get_g_main_context(IOThread *iothread) has to be called > > to run the gcontext in IOThread. If we use AioContext to dispatch socke= t > > fd events, we needn't to specify IOThread twice. Besides aio_poll is fa= ster > > than g_main_loop_run. > > > > Secondly, socket chardev's async read handler (set through > > qemu_chr_fe_set_handlers) doesn't take the case of socket short read > > into consideration. I plan to add one which will make use qio_channel_= yield. > > > > According to > > [1] Improving the QEMU Event Loop - Linux Foundation Events > > http://events17.linuxfoundation.org/sites/events/files/slides/Improving= %20the%20QEMU%20Event%20Loop%20-%203.pdf > > > > "Convert chardev GSource to aio or an equivalent source" (p.30) should = have > > been finished. I'm curious why the plan didn't continue. If it's desira= ble, > > I'm going to finish the leftover work to resolve the aforementioned two= issues. >=20 > Converting all chardevs to Aio might be challenging, and doesn't bring > much benefits imho. >=20 > Perhaps a better approach would be to rely on a new chardev API to > steal the chardev underlying fd or QIO... (mostly keeping -chardev for > CLI/QMP compatibility reason - although breaking some chardev features > that imho aren't compatible with all use cases, like replay, muxing, > swapping etc). The chardev should probably be removed after that... Yeah, I feel like it was a mistake for us to wire up many of our features to chardevs. We mostly did it because -chardev provides a pre-existing syntax for TCP/UNIX sockets and we didn't want to invent new CLI args. IMHO this was a mistake in retrospect. Unfortunately the -chardev API is absolutely terrible for any usage that actually cares about the connection based semantics. Witness the horrible hacks we do for re-connect and re-try when failing to initially connect in vhost-user net code. For features in QEMU where the only desirable chardev backend is one with connection based, socket semantics, I think we would be better off using the QIOChannel APIs directly and completely avoiding the chardev code. Regards, Daniel --=20 |: https://berrange.com -o- https://www.flickr.com/photos/dberrange= :| |: https://libvirt.org -o- https://fstop138.berrange.com= :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange= :|