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

["Rafael J. Wysocki" <rjw@xxxxxxx>]

On Monday, January 21, 2013 04:31:05 PM Borislav Petkov wrote:
> On Mon, Jan 21, 2013 at 03:03:43PM +0000, Stefan Bader wrote:
> > From 9870926d4a847e36c0f61921762fd50f1c92f75d Mon Sep 17 00:00:00 2001
> > From: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
> > Date: Mon, 14 Jan 2013 16:17:00 +0100
> > Subject: [PATCH] ACPI: Check MSR valid bit before using P-state frequencies
> > 
> > To fix incorrect P-state frequencies which can happen on
> > some AMD systems f594065faf4f9067c2283a34619fc0714e79a98d
> >   "ACPI: Add fixups for AMD P-state figures"
> > introduced a quirk to obtain the correct values by reading
> > from AMD specific MSRs.
> > 
> > This did cause a regression when running a kernel using that
> > quirk under Xen which does (currently) not pass through MSR
> > reads to the HW. Instead the guest gets a 0 in return.
> > And this seems to cause a failure to initialize the ondemand
> > governour (hard to say for sure as all P-states appear to run
> > at the same frequency).
> > 
> > While this should also be fixed in the hypervisor (to allow
> > a guest to read that MSR), this patch is intended to work
> > around the issue in the meantime. In discussion it turned out
> > that indeed real HW/BIOSes may choose to not set the valid bit
> > and thus mark the P-state as invalid. So this could be considered
> > a fix for broken BIOSes that also works around the issue on Xen.
> > 
> > [v2] Reword description text and use helper for bit index.
> > Signed-off-by: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx # v3.7..
> > ---
> >  drivers/acpi/processor_perflib.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> > diff --git a/drivers/acpi/processor_perflib.c 
> > b/drivers/acpi/processor_perflib.c
> > index 836bfe0..caa042e 100644
> > --- a/drivers/acpi/processor_perflib.c
> > +++ b/drivers/acpi/processor_perflib.c
> > @@ -340,6 +340,17 @@ static void amd_fixup_frequency(struct
> > acpi_processor_px *px, int i)
> >     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);
> > +           /*
> > +            * MSR C001_0064+:
> > +            * Bit 63: PstateEn. Read-write. 1=The P-state specified by
> > +            * this MSR is valid. 0=The P-state specified by this MSR is
> > +            * not valid. The purpose of this register is to indicate if
> > +            * the rest of the P-state information in the register is
> > +            * valid after a reset; it controls no hardware.
> > +            */
> 
> Maybe this comment is a but too long and it contains that idiotic
> processor manual speak :-). It should've contained only the first
> sentence: "PstateEn. If set, the P-state is valid."
> 
> But maybe Rafael could correct it while committing, no reason to resend
> for that only.
> 
> Acked-by: Borislav Petkov <bp@xxxxxxx>

Applied, the comment fixed up.

It's in my bleeding-edge branch for now and I'll move it to the linux-next
branch after build testing.

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.

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