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

[Stefan Bader <stefan.bader@canonical.com>]

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On 14.01.2013 17:34, Borislav Petkov wrote:
> On Mon, Jan 14, 2013 at 04:58:54PM +0100, Stefan Bader wrote:
>> 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@redhat.com>
>> 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;
> 
> I don't think that's the right change - this is fixing baremetal so that
> it works on xen. And besides, this code was in powernow-k8 before so I'm
> wondering why did it work then.

This actually only started to work when the xen-processor module got introduced
to provide acpi information to the hypervisor. If I remember correctly
powernow-k8 did fail.
For the way I did the fix: the AMD BIOS docs seemed to indicate that even for
bare metal bit 63 would say whether the values are valid. So I thought this is a
nice coincidence that under Xen with all 0 this matches that special case... ;)

-Stefan

> 
>>                 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
> 
> Adding the new Andre. :-) Andre, what did we do for powernow-k8 on xen
> so that the F10h 50MHz steps quirk would work?
> 
> Thanks.
> 



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