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Mon, 28 Sep 2020 14:21:16 +0200 (CEST) From: Markus Armbruster To: Kevin Wolf Subject: Re: [PATCH v7 07/13] monitor: Make current monitor a per-coroutine property References: <20200909151149.490589-1-kwolf@redhat.com> <20200909151149.490589-8-kwolf@redhat.com> <874ko0o0jv.fsf@dusky.pond.sub.org> <20200925152307.GF5731@linux.fritz.box> <87sgb2l4wb.fsf@dusky.pond.sub.org> <20200928104214.GE5451@linux.fritz.box> Date: Mon, 28 Sep 2020 14:21:16 +0200 In-Reply-To: <20200928104214.GE5451@linux.fritz.box> (Kevin Wolf's message of "Mon, 28 Sep 2020 12:42:14 +0200") Message-ID: <878scugkhv.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=216.205.24.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/09/28 03:29:35 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.687, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action 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: stefanha@redhat.com, marcandre.lureau@gmail.com, qemu-devel@nongnu.org, qemu-block@nongnu.org, dgilbert@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Kevin Wolf writes: > Am 28.09.2020 um 09:47 hat Markus Armbruster geschrieben: >> Kevin Wolf writes: >> >> > Am 14.09.2020 um 17:11 hat Markus Armbruster geschrieben: >> >> Kevin Wolf writes: >> >> >> >> > This way, a monitor command handler will still be able to access the >> >> > current monitor, but when it yields, all other code code will correctly >> >> > get NULL from monitor_cur(). >> >> > >> >> > This uses a hash table to map the coroutine pointer to the current >> >> > monitor of that coroutine. Outside of coroutine context, we associate >> >> > the current monitor with the leader coroutine of the current thread. >> >> >> >> In qemu-system-FOO, the hash table can have only these entries: >> >> >> >> * (OOB) One mapping @mon_iothread's thread leader to a QMP monitor, while >> >> executing a QMP command out-of-band. >> >> >> >> * (QMP-CO) One mapping @qmp_dispatcher_co (a coroutine in the main >> >> thread) to a QMP monitor, while executing a QMP command in-band and in >> >> coroutine context. >> >> >> >> * (QMP) One mapping the main thread's leader to a QMP monitor, while >> >> executing a QMP command in-band and out of coroutine context, in a >> >> bottom half. >> >> >> >> * (HMP) One mapping the main thread's leader to an HMP monitor, while >> >> executing an HMP command out of coroutine context. >> >> >> >> * (HMP-CO) One mapping a transient coroutine in the main thread to an >> >> HMP monitor, while executing an HMP command in coroutine context. >> >> >> >> In-band execution is one command after the other. >> >> >> >> Therefore, at most one monitor command can be executing in-band at any >> >> time. >> >> >> >> Therefore, the hash table has at most *two* entries: one (OOB), and one >> >> of the other four. >> >> >> >> Can you shoot any holes into my argument? >> > >> > I think with human-monitor-command, you can have three mappings: >> > >> > 1. The main thread's leader (it is a non-coroutine QMP command) to the >> > QMP monitor >> >> This is (QMP). >> >> > 2. With a coroutine HMP command, one mapping from the transient HMP >> > coroutine to the transient HMP monitor (with a non-coroutine HMP >> > command, we'd instead temporarily change the mapping from 1.) >> >> This is (HMP-CO). >> >> > 3. The OOB entry >> >> This is (OOB). >> >> To get 1. (QMP) and 2, (HMP-CO) at the same time, the in-band, >> non-coroutine QMP command needs to execute interleaved with the in-band, >> coroutine HMP command. >> >> Such an interleaving contradicts "In-band execution is one command after >> the other", which is a fundamental assumption in-band commands may make. >> If the assumption is invalid, we got a problem. Is it? > > Interleaving, or rather executing another command in the middle of its > implementation is the very purpose of human-monitor-command (which is > what I was talking about, so "the in-band non-coroutine QMP command" is > a very specific one). Got it now, thanks. > It's the only command I can think of that is exceptional in this way > and would lead to three mappings. > > Kevin