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

Stefan Bader
2013-01-21T15:03:56Z

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Ok, so how about this?

-Stefan

From 9870926d4a847e36c0f61921762fd50f1c92f75d Mon Sep 17 00:00:00 2001
From: Stefan Bader <stefan.bader@canonical.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@canonical.com>
Cc: stable@vger.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

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>From 9870926d4a847e36c0f61921762fd50f1c92f75d Mon Sep 17 00:00:00 2001
From: Stefan Bader <stefan.bader@canonical.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@canonical.com>
Cc: stable@vger.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