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

[Stefan Bader <stefan.bader@xxxxxxxxxxxxx>]

Ok, so how about this?

-Stefan

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.
+                */
+               if (!(hi & BIT(31)))
+                       return;
+
                fid = lo & 0x3f;
                did = (lo >> 6) & 7;
                if (boot_cpu_data.x86 == 0x10)
--
1.8.0

Attachment: 0001-ACPI-Check-MSR-valid-bit-before-using-P-state-freque.patch
Description: Text Data

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