From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeremy Fitzhardinge <jeremy@goop.org>
Subject: Re: [PATCH] RFC: Linux: disable APERF/MPERF feature in
	PV kernels
Date: Tue, 22 May 2012 09:52:27 -0700
Message-ID: <4FBBC44B.9020007@goop.org>
References: <4FBBB9AF.6020704@amd.com>
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To: Andre Przywara <andre.przywara@amd.com>
Cc: xen-devel <xen-devel@lists.xensource.com>, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
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On 05/22/2012 09:07 AM, Andre Przywara wrote:
> 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.

Seems reasonable to me.  Do all those RDMSR traps have a measurable
performance effect?

Also, is there a symbolic constant for that bit?

    J