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

["André Przywara" <andre@andrep.de>]

On Mon, 14 Jan 2013 18:08:45 +0100
Stefan Bader <stefan.bader@canonical.com> wrote:

> 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... ;)

>From a first glance I think this fix is a valid approach. There are
BIOSes which disable P-states via this bit, so we have to observe this
for bare-metal, too. Let me think a bit more about this, however, and
see whether there is a better solution to do the right thing (tm) under
Xen. Getting back to you then.

Thanks,
Andre.