Marc-André Lureau <marcandre.lur...@redhat.com> writes: > Hi > > On Thu, Dec 6, 2018 at 1:13 PM Markus Armbruster <arm...@redhat.com> wrote: >> >> Marc-André Lureau <marcandre.lur...@redhat.com> writes: >> >> > Hi >> > On Thu, Dec 6, 2018 at 10:08 AM Markus Armbruster <arm...@redhat.com> >> > wrote: >> >> >> >> One more question... >> >> >> >> Marc-André Lureau <marcandre.lur...@redhat.com> 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? > > The mux should be gcontext-feature neutral, or it should in fact > reflects the backend capability, since it is entirely driven by it.
Yes, that makes sense. > For now, it is simpler to keep it mark as unsupport, and I'll probably > update the aforementioned patch when resubmitting. Okay. >> > 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. > > Yes, as the context is associated with a thread. If a backend is not > able to switch context, it will keep dispatching in the default > context, which may have undesirable results for the frontend. > >> >> 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? > > For some reason, the chardev API let the frontend decide which context > should be used for the dispatch. > > This is quite fine when you have a one-to-one relationship between > backend and frontend (as long as the backend complies with context > switching, ie FEATURE_GCONTEXT). > > But in a one-to-many, as is the case with MUX, things get more > complicated, because one frontend may want to switch the context > (typically an oob monitor, moving dispatch to the iothread) while > another frontend (typically, a serial device) may not expect to be > dispatched from a different thread than the default thread. > > As you can see, MUX has two problems wrt context switching: backend > and frontends. Thanks, that helped some. > I think it would be safer to mark MUX as > !FEATURE_GCONTEXT (although in fact, you could use it if you really > now what you do with backend & frontends) There's no pressing need for a smarter chardev-mux that provides FEATURE_GCONTEXT in cases where it's safe. Simply not providing it at all is good enough. Testing CHARDEV_IS_MUX() in addition to FEATURE_GCONTEXT is then redundant. This makes me think we should drop the CHARDEV_IS_MUX() check from monitor_init(), and update the commit message to say We already forbid MUX because not all frontends are ready to run outside main loop. Replace that by a context-switching feature check. What do you think?