I am still having problems getting a graphical screen booting a xen pv guest with graphics using /usr/bin/qemu-system-i386 with qemu-2.6.0-4.fc24 (as expected the fix for Bug 1340931 doesn't fix the root problem). I have been trying to work out what is happening, and although I haven't found the problem I have some more clues. The output from qemu via xen when this failure happens is xen be core: xen be: watching backend path (backend/console/2) failed xen be core: xen be: watching backend path (backend/vkbd/2) failed xen be core: xen be: watching backend path (backend/vfb/2) failed xen be core: xen be: watching backend path (backend/qdisk/2) failed xen be core: xen be: watching backend path (backend/qnic/2) failed qemu-system-i386: terminating on signal 1 from pid 8157 so the there seems to be a problem with qemu connecting with the xenstored socket. Also if I build qemu-system-i386 with the --disable-rbd option added then then I do get a graphic screen again, so it seems to be some interaction between xen and rbd.
Have you reported this to xen-devel or similar? Devs there probably have a better chance of figuring this out...
I have identified the problem. The xs_watch function (called from xenstore_scan in hw/xen/xen_backend.c) creates a read pthread with an initial stack size of 16384, which is okay for qemu without rbd, but not enough if rbd is enabled. It does work if I increase the stack size setting to 24576. Do you know why there is an increased stack size requirement for pthreads in qemu with rbd?
No particular reason, just that rbd has pretty deep call stacks.
Sounds like this was eventually fixed in qemu: commit 66668d197fa40747e835e15617eda2f1bc80982f Author: Peter Lieven <pl> Date: Fri Jul 15 11:45:11 2016 +0200 vnc-tight: fix regression with libxenstore
qemu-2.6.2-8.fc24 has been submitted as an update to Fedora 24. https://bodhi.fedoraproject.org/updates/FEDORA-2017-374389c196
qemu-2.6.2-8.fc24 has been pushed to the Fedora 24 testing repository. If problems still persist, please make note of it in this bug report. See https://fedoraproject.org/wiki/QA:Updates_Testing for instructions on how to install test updates. You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2017-374389c196
qemu-2.6.2-8.fc24 has been pushed to the Fedora 24 stable repository. If problems still persist, please make note of it in this bug report.