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

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

On 05/23/2012 09:34 AM, Jan Beulich wrote:
>>>> On 22.05.12 at 18:07, Andre Przywara<andre.przywara@amd.com>  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.
>>
>> Awaiting your comments.
>
> First of all I'm of the opinion that this indeed should not be
> masked in the hypervisor - there's no reason to disallow the
> guest to read these registers (but we should of course deny
> writes as long as Xen is controlling P-states, which we do).

Ok. Thanks for the acknowledgment.

> Next I'd like to note that in our kernels we simply don't build
> arch/x86/kernel/cpu/sched.o. Together with CPU_FREQ being
> suppressed, there's no consumer of the feature flag in our
> kernels.

With "our kernels" you mean OpenSuSE/SLES kernels? I quickly checked 
upstream as well as the repos on kernel.opensuse.org. In all of them 
sched.o is unconditionally included in the Makefile.
So is there a build patch to exclude this file for builds of distro Xen 
kernels?

Regards,
Andre.


> So I would think that your suggested change is appropriate,
> but I'm adding Konrad to Cc as these days he's the one to pick
> this up.
>
> Jan

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