[Bug 1448985] Re: Ubuntu 14.04 LTS, 14.10, 15.04, 15.10 guests do not boot to Unity from QEMU-KVM Ubuntu 14.04 LTS, 14.10, 15.04 hosts

[Stefan Bader]

Finally with a little help on debugging unity, I think I see what is
going on. The obvious part seemed to be that compiz seems to fail
starting. Which turns out to be LLVM-pipe bailing with an obscure error
message:

LLVM ERROR: Do not know how to split the result of this operator!

This made me think about the emulated CPU, even more because Xen HVM
guests do work with the i386 isos. And really this is the actual
problem! So one quick work-around is to force qemu to use a different
CPU type. In my test I used the Wily i386 iso and changed the CPU type
from the default (QEMU Virtual CPU v2.0.0) into core2duo. And suddenly
the live session works.

So next, what is the difference. Beside of name, model, and stepping, it
seems the cpuid flags:

Core(TM)2 Duo CPU (emulated)    QEMU Virtual CPU v2.0.0
model 15                        model 6
stepping 11                     stepping 3
cpuid level 10                  cpuid level 4

ore(TM)2 Duo CPU (emulated)	QEMU Virtual CPU v2.0.0
model 15			model 6
stepping 11			stepping 3
cpuid level 10			cpuid level 4
 -->	fpu vme de pse tsc msr pae mce
	fpu         de pse tsc msr pae mce	<--
 -->	cx8 apic sep mtrr pge mca cmov
	cx8 apic sep mtrr pge mca cmov	<--
 -->	pat pse36 clflush mmx fxsr sse
	    pse36 clflush mmx fxsr sse	<--
 -->	sse2 ss syscall nx lm
	sse2    syscall nx lm		<--
 -->	constant_tsc pni vmx ssse3
	                        pni vmx		<--
 -->	cx16 x2apic               hypervisor
	cx16 x2apic popcnt hypervisor	<--
 -->	lahf_lm vnmi ept
	lahf_lm vnmi ept		<--

So QEMU CPU misses: vme, pat, ss, constant_tsc, and ssse3 but has popcnt. A different host running Xen was also missing ss, constant_tsc, ssse3, and popct. But it had vme and pat and was working with the i386 iso.
That sounds a bit like either vme or pat missing could cause the i386 build of llvmpipe to fail while the exact same cpuid configuration works with the amd64 version of llvmpipe.

-- 
You received this bug notification because you are a member of Ubuntu
Server Team, which is subscribed to qemu in Ubuntu.
https://bugs.launchpad.net/bugs/1448985

Title:
  Ubuntu 14.04 LTS, 14.10, 15.04, 15.10 guests do not boot to Unity from
  QEMU-KVM Ubuntu 14.04 LTS, 14.10, 15.04 hosts

To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1448985/+subscriptions