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=-0.6 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 8A276C2BB85 for ; Tue, 14 Apr 2020 07:27:05 +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 517EF20767 for ; Tue, 14 Apr 2020 07:27:05 +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="B7hfBccx" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 517EF20767 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]:53696 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jOFyK-0001xa-Fb for qemu-devel@archiver.kernel.org; Tue, 14 Apr 2020 03:27:04 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:43824) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jOFxZ-00014z-9S for qemu-devel@nongnu.org; Tue, 14 Apr 2020 03:26:18 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1jOFxX-0007Kp-QK for qemu-devel@nongnu.org; Tue, 14 Apr 2020 03:26:17 -0400 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:23113 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 1jOFxX-0007IX-MH for qemu-devel@nongnu.org; Tue, 14 Apr 2020 03:26:15 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1586849174; h=from:from: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=UI7VY/+zR2XVHCEjX835lS5VnXisM5MkifmQS9MTDNE=; b=B7hfBccxpeByzYAhZCbfQZI0Lr9YwwX8BnfZMB/HvYbN3JGlTQ0+mbwZj444TdUJOK3WG+ aauolIQydSpV2F/11riqzwj1IQ+p7JDe4E8+4FCoYV8kPWStoXRPuFR2h5ObCHYZR5rjka 3AaQ81vsC9SXtKyPxmzqyD4+iewtVvU= 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-147-y42YrLU-M9O7KkiqR3beRQ-1; Tue, 14 Apr 2020 03:26:07 -0400 X-MC-Unique: y42YrLU-M9O7KkiqR3beRQ-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 11B6C149C0; Tue, 14 Apr 2020 07:26:06 +0000 (UTC) Received: from blackfin.pond.sub.org (ovpn-113-20.ams2.redhat.com [10.36.113.20]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 6578996B93; Tue, 14 Apr 2020 07:26:00 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id A6D0B11385C8; Tue, 14 Apr 2020 09:25:58 +0200 (CEST) From: Markus Armbruster To: Daniel P. =?utf-8?Q?Berrang=C3=A9?= Subject: Re: Replace GSource with AioContext for chardev References: <20200409124601.toh6jpbfcwiwzb6z@r> <20200409132441.GS1202384@redhat.com> Date: Tue, 14 Apr 2020 09:25:58 +0200 In-Reply-To: <20200409132441.GS1202384@redhat.com> ("Daniel P. =?utf-8?Q?B?= =?utf-8?Q?errang=C3=A9=22's?= message of "Thu, 9 Apr 2020 14:24:41 +0100") Message-ID: <87imi2zfy1.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] 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: , Cc: Fam Zheng , Kevin Wolf , qemu-devel , Coiby Xu , Stefan Hajnoczi , Paolo Bonzini , =?utf-8?Q?Marc-Andr=C3=A9?= Lureau Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Daniel P. Berrang=C3=A9 writes: > 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,io= thread=3Diothread0 \ >> > -object vhost-user-blk-server,node-name=3Ddisk,chardev=3Dmon1,ioth= read=3Diothread0 \ >> > -object iothread,id=3Diothread0 \ >> > -chardev socket,id=3Dmon1,path=3D/tmp/vhost-user-blk_vhost.socket,= server,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 sock= et >> > fd events, we needn't to specify IOThread twice. Besides aio_poll is f= aster >> > 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/Improvin= g%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 desir= able, >> > I'm going to finish the leftover work to resolve the aforementioned tw= o 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. How do we get from here to there?