I'm running Xen 4.6.1 with Alpine Linux 3.3.3 in dom0. In a HVM domU 
with vga="qxl", Xorg will segfault instantly if tried started. Multiple 
Linux distros have been tested and Xorg segfaults in all.

Attached are a full backtrace from domU generated by Xorg, and a 
assembler dump of function 'sse2_blt'.

According to Xen IRC channel, the cause could be a bug in the x86 
instruction emulator related to SSE.