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

On 22/05/12 17:52, Jeremy Fitzhardinge wrote:
On 05/22/2012 09:07 AM, Andre Przywara wrote:

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?


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I've attached a patch which masks the matching CPUID leaves in the Xen pv_cpuid 
Should the logic in pv_cpuid be changed to only pass through explictly allowed 
CPUID leaves rather than masking
them using case statements?


Attachment: mask-thermal-and-power-management-leaf.patch
Description: Text Data

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