From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55703) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gUpjB-0002aW-IC for qemu-devel@nongnu.org; Thu, 06 Dec 2018 04:13:50 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gUpj6-0006SC-Hk for qemu-devel@nongnu.org; Thu, 06 Dec 2018 04:13:49 -0500 Received: from mx1.redhat.com ([209.132.183.28]:56344) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gUpj6-0006Ru-57 for qemu-devel@nongnu.org; Thu, 06 Dec 2018 04:13:44 -0500 From: Markus Armbruster References: <20181205203737.9011-1-marcandre.lureau@redhat.com> <20181205203737.9011-5-marcandre.lureau@redhat.com> <87bm5zkx0o.fsf@dusky.pond.sub.org> Date: Thu, 06 Dec 2018 10:13:35 +0100 In-Reply-To: (=?utf-8?Q?=22Marc-Andr=C3=A9?= Lureau"'s message of "Thu, 6 Dec 2018 10:23:45 +0400") Message-ID: <87tvjrhvb4.fsf@dusky.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH for-4.0 v4 4/7] monitor: check if chardev can switch gcontext for OOB List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: =?utf-8?Q?Marc-Andr=C3=A9?= Lureau Cc: lizhijian@cn.fujitsu.com, Jason Wang , "Dr. David Alan Gilbert" , Peter Xu , qemu-devel , zhangckid@gmail.com, "Bonzini, Paolo" Marc-Andr=C3=A9 Lureau writes: > Hi > On Thu, Dec 6, 2018 at 10:08 AM Markus Armbruster wro= te: >> >> One more question... >> >> Marc-Andr=C3=A9 Lureau writes: >> >> > Not all backends are able to switch gcontext. Those backends cannot >> > drive a OOB monitor (the monitor would then be blocking on main >> > thread). >> > >> > For example, ringbuf, spice, or more esoteric input chardevs like >> > braille or MUX. >> >> These chardevs don't provide QEMU_CHAR_FEATURE_GCONTEXT. >> >> > We currently forbid MUX because not all frontends are ready to run >> > outside main loop. Extend to add a context-switching feature check. >> >> Why check CHARDEV_IS_MUX() when chardev-mux already fails the >> qemu_char_feature_gcontext(chr, QEMU_CHAR_FEATURE_GCONTEXT) check? >> > > > It currently fails, but with "[PATCH 4/9] char: update the mux > hanlders in class callback", it won't. That's because it makes chardev-mux implement chr_update_read_handler(), and "[PATCH 3/7] char: add a QEMU_CHAR_FEATURE_GCONTEXT flag" assumes that a chardev implementing that "will take the updated gcontext into account". Sounds to me as if "[PATCH 4/9] char: update the mux hanlders in class callback" violates that assumption. Why am I wrong? > But the main reason to keep an explicit check on mux is that the > monitor frontend doesn't know if other mux frontends can be called > from any context (when you set a context, it is set on the backend > side, events are dispatched by the backend). > > We may want to mix this extra frontend-side capability limitation with > FEATURE_GCONTEXT flag, but they are fundamentally different: to be > able to set a backend context VS attached mux frontends can be > dispatched from any context. I'm afraid I can't yet see the full picture. The goal of this series PATCH 3-5 is to catch certain thread-related badness in chardevs before it can happen. Apparently, there are two separate kinds of badness: * The chardev backend may fail to cope with changed gcontext. I don't understand how exactly the backends screw up, but I doubt I have to right now. * The chardev frontend may fail to... what exactly? And why is only chardev-mux affected? [...]