[PATCH] RFC: Linux: disable APERF/MPERF feature in PV kernels

[Andre Przywara <andre.przywara@amd.com>]

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Hi,

while testing some APERF/MPERF semantics I discovered that this feature 
is enabled in Xen Dom0, but is not reliable.
The Linux kernel's scheduler uses this feature if it sees the CPUID bit, 
leading to costly RDMSR traps (a few 100,000s during a kernel compile) 
and bogus values due to VCPU migration during the measurement.
The attached patch explicitly disables this CPU capability inside the 
Linux kernel, I couldn't measure any APERF/MPERF reads anymore with the 
patch applied.
I am not sure if the PVOPS code is the right place to fix this, we could 
as well do it in the HV's xen/arch/x86/traps.c:pv_cpuid().
Also when the Dom0 VCPUs are pinned, we could allow this, but I am not 
sure if it's worth to do so.

Awaiting your comments.

Regards,
Andre.

P.S. Of course this doesn't fix pure userland software like cpupower, 
but I would consider this in the user's responsibility to not use these 
tools in Dom0, but instead use xenpm.

-- 
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany

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commit e802e47d85314b4541288e4a19d057e2ea885a28
Author: Andre Przywara <andre.przywara@amd.com>
Date:   Tue May 22 15:13:07 2012 +0200

    filter APERFMPERF feature in Xen to avoid kernel internal usage
    
    Signed-off-by: Andre Przywara <andre.przywara@amd.com>

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 95dccce..71252d5 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -240,6 +240,11 @@ static void xen_cpuid(unsigned int *ax, unsigned int *bx,
 		*dx = cpuid_leaf5_edx_val;
 		return;
 
+	case 6:
+		/* Disabling APERFMPERF for kernel usage */
+		maskecx = ~(1U << 0);
+		break;
+
 	case 0xb:
 		/* Suppress extended topology stuff */
 		maskebx = 0;

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