I also see this error on 15.04, running 3.19.0-23-generic in a Xen DomU. I had to patch the radeon driver in the kernel in order for it to work correctly with passthrough. I will take a closer look at that patch, but it is pretty straightforward.
The relevant portion of Xorg.0.log: [ 14.281] (==) FBDEV(0): Backing store enabled [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.281] (EE) FBDEV(0): FBIOPUTCMAP: Device or resource busy [ 14.282] (==) FBDEV(0): DPMS enabled [ 14.282] (--) RandR disabled [ 14.285] (II) SELinux: Disabled on system [ 14.286] (II) AIGLX: Screen 0 is not DRI2 capable [ 14.286] (EE) AIGLX: reverting to software rendering [ 14.287] (II) AIGLX: Loaded and initialized swrast [ 14.287] (II) GLX: Initialized DRISWRAST GL provider for screen 0 [ 14.324] (EE) [ 14.324] (EE) Backtrace: [ 14.324] (EE) 0: /usr/bin/X (xorg_backtrace+0x56) [0x7f24ca63e556] [ 14.324] (EE) 1: /usr/bin/X (0x7f24ca48b000+0x1b7749) [0x7f24ca642749] [ 14.324] (EE) 2: /lib/x86_64-linux-gnu/libc.so.6 (0x7f24c8151000+0x352f0) [0x7f24c81862f0] [ 14.324] (EE) 3: /usr/bin/X (RRSetChanged+0x50) [0x7f24ca5a26b0] [ 14.324] (EE) 4: /usr/bin/X (RRScreenSetSizeRange+0x54) [0x7f24ca5a6be4] [ 14.324] (EE) 5: /usr/bin/X (xf86RandR12CreateScreenResources+0x2a5) [0x7f24ca563b75] [ 14.324] (EE) 6: /usr/bin/X (0x7f24ca48b000+0xcbe10) [0x7f24ca556e10] [ 14.324] (EE) 7: /usr/bin/X (0x7f24ca48b000+0x5c104) [0x7f24ca4e7104] [ 14.324] (EE) 8: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xf0) [0x7f24c8171a40] [ 14.324] (EE) 9: /usr/bin/X (0x7f24ca48b000+0x4662e) [0x7f24ca4d162e] [ 14.324] (EE) [ 14.324] (EE) Segmentation fault at address 0xa0 [ 14.324] (EE) Fatal server error: [ 14.324] (EE) Caught signal 11 (Segmentation fault). Server aborting [ 14.324] (EE) [ 14.324] (EE) -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1463224 Title: /usr/bin/X segfaults on startup To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/lightdm/+bug/1463224/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs