Comment 18 for bug 1038332

Tim,

Thanks for posting it in the Xen BZ. The issue is .. well, it is a long explanation, so skip to the end for summary if you don't want to read technical jargon.

On AMD with Xen, the cpuidle is set to use the halt one (this is b/c the halt ends up doing a yield hypercall) - look in setup.c - and the hypervisor does the appropiate halt operation (MWAIT, halt, etc, or schedules another guest on the CPU). Anyhow, to not have the cpuidle trying to activate, the "boot_option_idle_override" is set. Therefore, the ACPI _PSS driver (processor.ko) ends up bailing out, b/c of that parameter. As such the "older" AMD pstate driver is invoked (powernow-k8), and the older driver attempts to use ACPI _PSD - but only if in UP mode, or it attempts to use the voltage tables - which are k8 or earlier. To detect that, it use the MSR (sadly not CPUID values), which Xen traps and returns 00, which the powernow-k8 driver interprets as "buggy hardware - can't use". Which is exactly what you are seeing.

I believe (and sadly I don't have the hardware to check this - but I think I saw the somebody using it) if you were running on K8 hardware - it ought to work.

Solution: Have the ACPI processor driver cooperate with Xen. Patches are in the queue for it (if you are really interested look in oss.oracle.com/kwilk/xen.git #devel/acpi-cpufreq.v2 - but they are not yet upstream-able material. Actually, they are quite ugly).

Other solution:
The "easy" option for right now would be to do what Dave suggest until the upstream patches are ready and reviewed.