RE: [Xen-devel][PATCH 0/5] Add cpufreq pwr mgmt to Xen

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

>From: Langsdorf, Mark [mailto:mark.langsdorf@xxxxxxx] 
>Sent: 2008年5月16日 5:55
>
>> The following patchset is to add processor P state power management
>> support to Xen X86.
>
>Is there any way to determine what frequencies the
>CPU is running at now?  I think I have support for
>the AMD Architectural P-state driver working now,
>but it would be nice if there was a better way to
>test that it's working.
>

Mark, the efficient way to check current freq is cpu specific.
ACPI defines a way to read PERF_STATUS and then
compare to _PSS status field as confirmation whether the
transition succeeds. Corresponding MSR in Intel are
IA32_PERF_STATUS upon which cpufreq driver exacts to
compare freq table for actual value. As hardware coordination
is active, above value doesn't reflect the actual freq. Thus
there're a new MSR pair (MPERF/APERF) on Intel processors
to report the actual freq. You can check get_cur_freq_on_cpu
for all possible ways. I can't tell for AMD cpus but I guess
similar MSRs may also exist in your case.

Thanks,
Kevin

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