Paolo Bonzini <pbonz...@redhat.com> writes: > Il 04/04/2013 18:59, Anthony Liguori ha scritto: >>> > >>> > The right thing to use would be g_source_add_child_source() and >>> > g_source_remove_child_source(), but that is only present since glib 2.28 >>> > and we currently require 2.12 (2.20 on Windows). >> I don't think a child source fixes the problem. The backend definitely >> has work to do. What we don't know is whether the front end is capable >> of processing the work. >> >> The problem here is that we use polling on the front-end. IOW: >> >> 1) Char backend has data and is ready to write. >> 2) Asks front end if it can write >> 3) Front end says no >> 4) Goto (1) > > What we used to do is this, however: > > 1) Char backend asks front end if it can write > 2) Front end says no
You're missing a step here. Previously we would drop the data silently. > 3) poll() goes on without char backend's descriptor Which is what enabled this. > 4) Goto (1), then: > 5) Front end is now available, calls qemu_notify_event() This still exists FWIW. > 6) Char backend asks front end if it can write > 7) Front end says yes > > No busy waiting here. Though we don't really do (5) everywhere, because > for example this patch slipped through the cracks: Ack. > http://lists.gnu.org/archive/html/qemu-devel/2012-03/msg03208.html (but > for the monitor and every consumer running in iothread context it's ok). > > You could achieve the same thing by making the io watch a child source > of the QEMU wrapper. All the wrapper does is add/remove the io watch > source from its children, depending on can_read. I don't understand how this solves the problem. There aren't two sources. There is only a single source (the GIOChannel source). We override the function pointers to basically do monkey patching of the GSource. If I understand your suggestion, it's to add the source as a child of itself. > > Perhaps it's possible without child sources though, by setting the > callbacks on the glib source directly. That's what we're doing... src = g_io_create_watch(channel, G_IO_IN | G_IO_ERR | G_IO_HUP); g_source_set_funcs(src, &io_watch_poll_funcs); g_source_set_callback(src, (GSourceFunc)fd_read, user_data, NULL); There is only one GSource active here. Regards, Anthony Liguori > Paolo
