[Qemu-devel] tainted Linux kernel in default SMP QEMU/KVM guests

[Andre Przywara]

Hi,

since the default CPU model for QEMU (qemu64) is an AMD K7, the Linux 
kernel complains and taints the kernel when it detects multiple 
processors. The reason for this is a check for SMP safe CPUs in 
arch/x86/kernel/cpu/amd.c:amd_k7_smp_check(). In recent kernels (since 
about 2.6.29) this in only executed in 32bit kernels, since the check 
has been #ifdef'ed CONFIG_X86_32.
The failing check triggers a nasty dump along with a call trace and 
taints the kernel, this confuses users, so I want to get rid of it.
One can work around it (using -cpu kvm64 or -cpu host), but I want to 
avoid it in the default case, too.

I see these possible solutions:

1) Change the default CPUID bits from 6/2/3 to 6/6/1, this passes the 
Linux kernel check. But I am not sure if that would introduce 
regressions, since some OSes apply quirks if they detect certain models 
(like we had with the sysenter issue in the past)

2) Only change the CPUID bits to 6/6/1 if we use SMP. Still has the 
above drawback, but would be limited to SMP guests only.

3) Set kvm64/kvm32 as the default CPU model if KVM is enabled. This 
would limit the report and taint to TCG, where SMP is rarely used. 
Additionally less people (if any) use it for production systems.

4) Make the Linux' kernel quirk dependent on the missing hypervisor bit. 
I don't think this will be accepted easily upstream (and I don't want to 
support Ingo's recent ideas ;-), also this would not fix older kernels.

I can easily provide patches for all solutions, but I'd like to get 
advice from people on which one to pursue.

Regards,
Andre.

 --
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 448-3567-12