Commit 5e626254 authored May 29, 2012 by Andre Przywara Committed by Konrad Rzeszutek Wilk May 31, 2012

xen/setup: filter APERFMPERF cpuid feature out



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 <andre.przywara@amd.com>
Cc: stable@vger.kernel.org # v3.0+
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

parent 58b7b53a

arch/x86/xen/enlighten.c

+8 −0

Original line number	Diff line number	Diff line
		@@ -208,6 +208,9 @@ 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;

		@@ -241,6 +244,11 @@ static void xen_cpuid(unsigned int ax, unsigned int bx,
		*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;

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