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Re: [Xen-devel] cpufreq status information

To: Jan Beulich <jbeulich@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] cpufreq status information
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Mon, 08 Sep 2008 14:38:41 +0100
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On 8/9/08 14:12, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

> Trying to understand whether CPU frequency scaling is actually working on
> a system currently requires (afaics) source patches, as there is no way to
> get the current state of a CPU. Even if this is intentional, this doesn't seem
> very helpful when considering to make this functionality available to
> customers: I'm certain quite a few will ask how they can tell whether this
> is actually working.

Well, they might notice that their battery lasts longer than an hour. :-)

> So I'm basically considering to add a generic mechanism first, and then
> make cpufreq the first user of it. The question just is - use a completely
> new (guest-ro) per-vCPU page, perhaps with chained descriptors rather
> than a fixed layout, or extend the vCPU info structure, but e.g. require
> the guest to use VCPUOP_register_vcpu_info to gain access to all
> structure fields.

I'm skeptical about throwing in more shared-memory structures between Xen
and guests anyway. It doesn't even seem a good fit here since this info is
not of great interest to most guests in my opinion. I presume most users'
curiosities will be satisfied by being able to poll CPU frequencies/voltages
from dom0 via a hypercall.

 -- Keir

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