RE: [Xen-devel] cpufreq support status

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

Attached patch allows dom0 to touch P-state control MSRs on Intel 
processors. As Keir said earlier, I/O space style doesn't need special 
handle since by default dom0 can access I/O ports unless explicitly 
denied.

A correction to earlier comment. Based on processor stepping, users 
have to try both acpi_cpufreq (for I/O) and speedstep_centrino (for 
MSR) drivers to see which one works, when xen-linux still stays at 
2.6.18. But once xen-linux syncs to latest kernel version, acpi_cpufreq 
then becomes the default one for both cases.

But at the same time, I would not suggest enabling on-demand governor 
by default before confirming its correctness.

Thanks,
Kevin

>From: Tian, Kevin
>Sent: 2007年10月24日 16:39
>
>>From: Jan Beulich
>>Sent: 2007年10月24日 16:28
>>
>>Looking into this some more, I find that
>>ACPI_ADR_SPACE_FIXED_HARDWARE
>>is mostly unsupported in Linux except for C-state handling, in which
>case
>>is
>>resolves to MWAIT.
>>
>
>FFH is mapped to msr for P-state handling on Intel processors. You
>can refer to acpi-cpufreq.c in latest native Linux kernel, where have
>logic to handle both IO space and FFH space.
>
>Since 2.6.2*, acpi-cpufreq.c is the preferred cpufreq driver for
>intel processors.
>
>But if you look at the current 2.6.18-xen, acpi-cpufreq.c can only handle
>I/O space and instead speedstep-centrino.c manipulates FFH, i.e. MSR
>directly. But because the _PDC capabilities is set to FFH capable in
>2.6.18, speedstep_centrino.c becomes the only drive working in this
>environment then.
>
>Thanks,
>Kevin
>
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Attachment: eist_msr.patch
Description: eist_msr.patch

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