Re: CVS commit: src/sys

[Christoph Egger <Christoph_Egger%gmx.de@localhost>]

On 08.08.10 19:08, Jukka Ruohonen wrote:
> On Sun, Aug 08, 2010 at 04:58:42PM +0000, Jukka Ruohonen wrote:
>> Merge P-state support for acpicpu(4).
> 
> Hi.
> 
> I'll post this note about conclusions from various informal discussions
> regarding acpicpu(4) and CPU power management generally.
> 
> In my opinion, it is increasingly evident that we will need a well-defined,
> machine-independent interface and KPI especially for CPU frequency
> management. I am very much willing to work with such a KPI, but the goal
> here has been to first get the ACPI implementation ready, as a stand-alone
> and modular component with minimal changes to existing MD code.
> 
> There is still much work to do with the ACPI CPU driver. But once it is
> ready and robust, we can continue to evaluate how much of this is
> x86-specific, what would be the API requirements, what potential efficiency
> concerns need to be taken into account, how CPU power management might be
> driven by the scheduler, and so forth.
> 
> So the MD things done here are more or less kludges to patch up things that 
> were already more or less kludges. The existing PowerNow! and EST need to
> be eventually rewritten from scratch.  The backwards compatibility need to
> broken at some point.  The sysctl(8) solution and user space "governors"
> need to be deprecated to reach the full potential of modern CPU power
> management on x86 and presumably on other architectures as well.

I want to bring up the way to go for Xen:

A Xen guest (both Dom0 and DomU) only see virtual cpus. Only the
hypervisor itself see the physical cpus.
Hence the C- and P-State handling must be done through hypercalls.
The hypercalls are for Dom0 only.
The hypervisor informs Dom0 via events about the physical cpu
frequencies so that Dom0 has a chance to react when changes happen.

Christoph