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

[Matt Wilson <msw@xxxxxxxxxx>]

On Tue, Jan 15, 2013 at 02:04:30PM +0100, Matt Wilson 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@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:
> 
> The correct values should be returned already via rdmsr if
> "cpureq=dom0-kernel" is specified on the Xen command line. Looking at
> the LP report, it doesn't seem that this option was used.
> 
> Likely you will also need to use "dom0_vcpu_pin=1" if you want dom0 to
> be the cpufreq controller on AMD.

Aah, after chatting with Konrad I understand the problem better. This
broke using Xen as the cpufreq controller - sorry for failing to
notice that. :-)

Checking for a valid result seems to make sense.

Matt

> > --- 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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