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

[Jeremy Fitzhardinge <jeremy@goop.org>]

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