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? > 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? [...]
