This is a note to let you know that I've just added the patch titled
xen/setup: filter APERFMPERF cpuid feature out
to the 3.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
xen-setup-filter-aperfmperf-cpuid-feature-out.patch
and it can be found in the queue-3.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <[email protected]> know about it.
>From 5e626254206a709c6e937f3dda69bf26c7344f6f Mon Sep 17 00:00:00 2001
From: Andre Przywara <[email protected]>
Date: Tue, 29 May 2012 13:07:31 +0200
Subject: xen/setup: filter APERFMPERF cpuid feature out
From: Andre Przywara <[email protected]>
commit 5e626254206a709c6e937f3dda69bf26c7344f6f upstream.
Xen PV kernels allow access to the APERF/MPERF registers to read the
effective frequency. Access to the MSRs is however redirected to the
currently scheduled physical CPU, making consecutive read and
compares unreliable. In addition each rdmsr traps into the hypervisor.
So to avoid bogus readouts and expensive traps, disable the kernel
internal feature flag for APERF/MPERF if running under Xen.
This will
a) remove the aperfmperf flag from /proc/cpuinfo
b) not mislead the power scheduler (arch/x86/kernel/cpu/sched.c) to
use the feature to improve scheduling (by default disabled)
c) not mislead the cpufreq driver to use the MSRs
This does not cover userland programs which access the MSRs via the
device file interface, but this will be addressed separately.
Signed-off-by: Andre Przywara <[email protected]>
Signed-off-by: Konrad Rzeszutek Wilk <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
arch/x86/xen/enlighten.c | 8 ++++++++
1 file changed, 8 insertions(+)
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -207,6 +207,9 @@ static void __init xen_banner(void)
xen_feature(XENFEAT_mmu_pt_update_preserve_ad) ? "
(preserve-AD)" : "");
}
+#define CPUID_THERM_POWER_LEAF 6
+#define APERFMPERF_PRESENT 0
+
static __read_mostly unsigned int cpuid_leaf1_edx_mask = ~0;
static __read_mostly unsigned int cpuid_leaf1_ecx_mask = ~0;
@@ -240,6 +243,11 @@ static void xen_cpuid(unsigned int *ax,
*dx = cpuid_leaf5_edx_val;
return;
+ case CPUID_THERM_POWER_LEAF:
+ /* Disabling APERFMPERF for kernel usage */
+ maskecx = ~(1 << APERFMPERF_PRESENT);
+ break;
+
case 0xb:
/* Suppress extended topology stuff */
maskebx = 0;
Patches currently in stable-queue which might be from [email protected] are
queue-3.4/xen-setup-filter-aperfmperf-cpuid-feature-out.patch
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