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

On 01/18/2013 08:03 PM, Borislav Petkov wrote:
On Fri, Jan 18, 2013 at 02:00:15PM -0500, Konrad Rzeszutek Wilk wrote:
I did not explain myself well. The fix is OK - it just that the
hypervisor causes the quirk to not work correctly. Hmm, I wonder if
there BIOSes that do the same thing (cause the MSR to return 0). Per
you estimation of BIOS quality, it seems that this could happen.

Yeah, I don't think there's a limit to the amount of SNAFU a BIOS can
cause :-).

So for having the "check for sensible BIOS" in mainline I refreshed the patch (fixed the bit test, and actually tested it this time) and also added some hopefully sensible explanation to it (attached below).

Should I send it to acpi lists or would that have to go via an Andre?


From 6e2fc8291c91339123a37162382d8b08b50867ba 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 on the contents
of the HW but 0. 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.

Signed-off-by: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v3.7..
 drivers/acpi/processor_perflib.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/acpi/processor_perflib.c b/drivers/acpi/processor_perflib.c
index 836bfe0..41f4bdac 100644
--- 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, 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);
+               /* Bit 63 indicates whether contents are valid */
+               if (!(hi & 0x80000000))
+                       return;
                fid = lo & 0x3f;
                did = (lo >> 6) & 7;
                if (boot_cpu_data.x86 == 0x10)

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

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