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Thu, 30 Jul 2020 15:59:42 +0000 (UTC) Date: Thu, 30 Jul 2020 16:59:39 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Maydell Subject: Re: [PATCH] osdep.h: Add doc comment for qemu_get_thread_id() Message-ID: <20200730155939.GP3477223@redhat.com> References: <20200716154114.10838-1-peter.maydell@linaro.org> <0f8b8fea-2bd0-7616-292b-8fb0f87cec75@redhat.com> <87k0ylvy0t.fsf@dusky.pond.sub.org> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/1.14.5 (2020-06-23) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Received-SPF: pass client-ip=207.211.31.120; envelope-from=berrange@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/30 03:59:04 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -40 X-Spam_score: -4.1 X-Spam_bar: ---- X-Spam_report: (-4.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, 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_H2=-1, SPF_HELO_NONE=0.001, T_SPF_TEMPERROR=0.01, URIBL_BLOCKED=0.001 autolearn=unavailable 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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: QEMU Trivial , Markus Armbruster , QEMU Developers Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Jul 30, 2020 at 04:52:49PM +0100, Peter Maydell wrote: > On Thu, 30 Jul 2020 at 16:11, Markus Armbruster wrote: > > > > Peter Maydell writes: > > > > > On Tue, 28 Jul 2020 at 16:17, Eric Blake wrote: > > >> > > >> On 7/16/20 10:41 AM, Peter Maydell wrote: > > >> > Add a documentation comment for qemu_get_thread_id(): since this > > >> > is rather host-OS-specific it's useful if people writing the > > >> > implementation and people thinking of using the function know > > >> > what the purpose and limitations are. > > >> > > > >> > Signed-off-by: Peter Maydell > > >> > --- > > >> > Based on conversation with Dan on IRC, and prompted by the recent > > >> > patch to add OpenBSD support. > > >> > > > >> > Q: should we document exactly what the thread-id value is for > > >> > each host platform in the QMP documentation ? Somebody writing > > >> > a management layer app should ideally not have to grovel through > > >> > the application to figure out what they should do with the > > >> > integer value they get back from query-cpus... > > >> > > > >> > include/qemu/osdep.h | 14 ++++++++++++++ > > >> > 1 file changed, 14 insertions(+) > > >> > > >> Do we need a counterpart change... > > >> > > >> > > > >> > diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h > > >> > index 4841b5c6b5f..8279f72e5ed 100644 > > >> > --- a/include/qemu/osdep.h > > >> > +++ b/include/qemu/osdep.h > > >> > @@ -515,6 +515,20 @@ bool qemu_has_ofd_lock(void); > > >> > > > >> > bool qemu_write_pidfile(const char *pidfile, Error **errp); > > >> > > > >> > +/** > > >> > + * qemu_get_thread_id: Return OS-specific ID of current thread > > >> > + * > > >> > + * This function returns an OS-specific identifier of the > > >> > + * current thread. This will be used for the "thread-id" field in > > >> > + * the response to the QMP query-cpus and query-iothreads commands. > > >> > > >> ...to the qapi definition of query-cpus and query-iothreads? > > > > > > Well, that was my question above. Currently the QAPI documentation > > > says absolutely nothing about what the thread-id values mean > > > for any host OS (beyond "ID of the underlying host thread"), which > > > means that any management layer application needs to look in the > > > implementation to find out what they actually are... > > > > ... which they will have to do to actually use it for the purpose we > > have in mind, namely: > > > > >> > + * The intention is that a VM management layer application can then > > >> > + * use it to tie specific QEMU vCPU and IO threads to specific host > > >> > + * CPUs using whatever the host OS's CPU affinity setting API is. > > >> > + * New implementations of this function for new host OSes should > > >> > + * return the most sensible integer ID that works for that purpose. > > >> > + * > > >> > + * This function should not be used for anything else inside QEMU. > > >> > + */ > > > > Do they? > > Well, I suspect that management-layer code currently has > gone for "assume we're always running on Linux" and was > written by people who knew they were getting a Linux tid... Yes, on the libvirt side, the functionality that relies on thread_is is only compiled on Linux. If someone wants to use it on other OS, they'll have to provide an impl using their platforms equivalent of sched_setaffinity and friends since none of this stuff is standardized across OS. > > The PID is quite unlikely to be "an OS-specific identifier of the > > current thread". Shouldn't we fail instead of lie when we don't know > > how to compute the truth? > > Yeah, I think the default codepath is pretty bogus too. Should > the QMP functions have a mechanism for saying "we don't know > a thread-id on this platform" ? Thread_id should be optional and thus not filled in if we can't provide a sensible value. Unfortunately we made it mandatory in QMP. Regards, Daniel -- |: 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 :|