linux-kernel - Re: kernel 3.7+ cpufreq regression on AMD system running as dom0

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Date:	Mon, 21 Jan 2013 15:03:43 +0000
From:	Stefan Bader <stefan.bader@...onical.com>
To:	Borislav Petkov <bp@...en8.de>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Andre Przywara <andre@...rep.de>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Matthew Garrett <mjg@...hat.com>
Subject: Re: kernel 3.7+ cpufreq regression on AMD system running as dom0

Ok, so how about this?

-Stefan

 From 9870926d4a847e36c0f61921762fd50f1c92f75d Mon Sep 17 00:00:00 2001
From: Stefan Bader <stefan.bader@...onical.com>
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@...onical.com>
Cc: stable@...r.kernel.org # 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

View attachment "0001-ACPI-Check-MSR-valid-bit-before-using-P-state-freque.patch" of type "text/x-diff" (2321 bytes)