On Thu, 03 Apr 2014 12:28:27 -0600 Alex Williamson <alex.williamson@xxxxxxxxxx> wrote: > I just happened to write-up an explanation this morning: > > https://bbs.archlinux.org/viewtopic.php?pid=1400212#p1400212 > > Last I heard DRI gets disabled whenever Xorg finds that the VGA > arbiter is managing multiple devices. Therefore, while X still > works, the typical use case will have a pretty severe performance > decrease. > > I started designing a next-gen VGA arbiter that provides userspace > with a file descriptor they can use for accessing VGA space, > including mmap. This seems to work, invalidating the mapping in the > no-lock-holder, allowing it to fault and block re-mapping until the > lock can be re-acquired. I got stalled trying to figure out how to > make use of it in Xorg though. If anyone is interested in working on > this, I'll happily post my new arbiter code. > > Dave Airlie also suggested that maybe all of the VGA arbiter support > in Xorg could be ripped out without anyone noticing (now I've spilled > the beans), but that would also solve the DRI problem. We'd probably > need to keep things as-is in i915 for compatibility for some time > though. > > That brings up another option of including those patches under some > kind of CONFIG_I915_DONT_LIE_TO_VGA_ARBITER option so a user could > configure their kernel how they choose. This may just turn the > complaints into "I want both" though. > > As noted in the forum, another option is to avoid VGA in the guest by > using a "legacy-free" UEFI BIOS. This may circumvent other problems, > like lack of arbiter support in other drivers, but it relies on a > compatible legacy-free graphics card BIOS and guest support. > > I'd certainly like to make progress on this, so welcome some sort of > discussion. Thanks, > > Alex > I did some more tests and managed to reproduce the issue, it seems to happen only if you use multiple devices for the same xserver: intel only xorg = OK nvidia vfio, intel xorg = OK nvidia only xorg = OK nvidia :0, intel :1 = OK nvidia :0.0, intel :0.1 = glxinfo claims everything's OK but glxgears segfaults Your next-gen VGA arbiter sounds great, but it also sounds very complicated and requiring a lot of work and testing, since you need to modify the kernel, xorg and also possibly wayland. Perhaps we could enhance your CONFIG_I915_DONT_LIE_TO_VGA_ARBITER idea as a short term solution: If it's a config option most distro makes would probably disable it after reading the help text, therefore I'd go for a module parameter, disabled by default. This would save the average user from recompiling (and possibly patching) the kernel and make using VFIO as easy as adding "i915.arbiter=1" to /etc/default/grub. This would also allow the "I want both" guys to unload and reload the module to change the behaviour. Regards, Friedrich _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
