[Xen-devel] kernel 3.7+ cpufreq regression on AMD system running as dom0

[Stefan Bader <stefan.bader@xxxxxxxxxxxxx>]

Starting with kernel v3.7 the following commit added a quirk
to obtain the real frequencies of certain AMD systems:

commit f594065faf4f9067c2283a34619fc0714e79a98d
Author: Matthew Garrett <mjg@xxxxxxxxxx>
Date:   Tue Sep 4 08:28:06 2012 +0000

    ACPI: Add fixups for AMD P-state figures

When running bare-metal, on my Opteron 6128 test box results
in the frequencies remaining effectively unchanged:
[    5.475735] P0: MSR(hi,lo): 8000015c-50004004
[    5.479049] P0: fid=0x4, did=0x0, freq: 2000 -> 2000
[    5.484001] P1: MSR(hi,lo): 8000014c-50004a4e
[    5.487314] P1: fid=0xe, did=0x1, freq: 1500 -> 1500
[    5.492272] P2: MSR(hi,lo): 80000141-50005048
[    5.495584] P2: fid=0x8, did=0x1, freq: 1200 -> 1200
[    5.500540] P3: MSR(hi,lo): 80000138-50005844
[    5.503853] P3: fid=0x4, did=0x1, freq: 1000 -> 1000
[    5.508812] P4: MSR(hi,lo): 80000131-50005c40
[    5.512125] P4: fid=0x0, did=0x1, freq: 800 -> 800

However running as dom0 under Xen 4.2, reading this MSR returns
null:
[   11.613068] P0: MSR(hi,lo): 00000000-00000000
[   11.613074] P0: fid=0x0, did=0x0, freq: 2000 -> 1600
[   11.613078] P1: MSR(hi,lo): 00000000-00000000
[   11.613081] P1: fid=0x0, did=0x0, freq: 1500 -> 1600
[   11.613085] P2: MSR(hi,lo): 00000000-00000000
[   11.613088] P2: fid=0x0, did=0x0, freq: 1200 -> 1600
[   11.613091] P3: MSR(hi,lo): 00000000-00000000
[   11.613094] P3: fid=0x0, did=0x0, freq: 1000 -> 1600
[   11.613098] P4: MSR(hi,lo): 00000000-00000000
[   11.613101] P4: fid=0x0, did=0x0, freq: 800 -> 1600

And this results in Xen failing to change the governor:
  "(XEN) Fail change to ondemand governor"

I suppose this ultimately requires some support in the hypervisor
to pass through the real values. But since this is at least on my
combination of Xen 4.2 + kernel v3.7+ and AMD family 0x10 CPU a
regression compared to older kernels, I wonder whether the following
change might be something that should go into mainline:

--- a/drivers/acpi/processor_perflib.c
+++ b/drivers/acpi/processor_perflib.c
@@ -340,6 +340,9 @@ static void amd_fixup_frequency(struct acpi_processor_px *px
        if ((boot_cpu_data.x86 == 0x10 && boot_cpu_data.x86_model < 10)
            || boot_cpu_data.x86 == 0x11) {
                rdmsr(MSR_AMD_PSTATE_DEF_BASE + index, lo, hi);
+               /* Bit 63 indicates whether contents are valid */
+               if (!(hi & 0x8000000))
+                       return;
                fid = lo & 0x3f;
                did = (lo >> 6) & 7;
                if (boot_cpu_data.x86 == 0x10)

I tested something similar (so hopefully I have not failed on slapping
together a cleaned up version), which did resolve the problem.

-Stefan

Reference: https://bugs.launchpad.net/bugs/1078619

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