Re: [Xen-devel] cpufreq support status

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 17/10/07 16:11, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

>> It's definitely k8-powernow only right now. acpi-cpufreq might work if the
>> control register is an i/o port. It won't work if the control register is an
>> MSR, since the MSR would not be whitelisted in the WRMSR emulation in
>> emulate_privileged_op().
> 
> Does ACPI have means to specify MSRs as the access mechanism? I don't
> recall platform specific behavior like this, I thought this was restricted to
> the set of types named ACPI_ADR_SPACE_* in Linux' include/acpi/actypes.h
> (and which already goes beyond what is in ACPI 3.0b).

Well, kind of but not really. ACPI_ADR_SPACE_FIXED_HARDWARE is used, and
this is a cop-out in the ACPI spec which means 'the OS needs extra info from
the CPU vendor'. So typically for power management it means that CPUID must
be mined to discover whether e.g., PowerNow or EnhancedSpeedStep MSRs should
be used.

This effectively means that the range of possible power-management MSRs is
very limited (since they cannot be specified dynamically in the ACPI tables)
and Xen itself should easily be able to probe for and dynamically white-list
available MSRs as appropriate. Probably all that needs to be added is
support for EnhancedSpeedStep MSRs, since we already have PowerNow support.

 -- Keir



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