Re: [Xen-devel] CPU frequency throttling based on the temperature

[Jan Beulich <JBeulich@xxxxxxxx>]

On 25.07.2019 15:47, Roger Pau Monné  wrote:
> On Thu, Jul 25, 2019 at 09:29:01AM -0400, Fredy P. wrote:
>> On Thu, 2019-07-25 at 15:13 +0200, Roger Pau Monné wrote:
>>> On Thu, Jul 25, 2019 at 12:54:46PM +0000, Jan Beulich wrote:
>>>> On 25.07.2019 14:44,  Fredy P.  wrote:
>>>>> On Wed, 2019-07-24 at 17:41 +0200, Roger Pau Monné wrote:
>>>>>>>> What hardware interface does thermald (or the driver in
>>>>>>>> Linux if
>>>>>>>> there's one) use to get the temperature data?
>>>>>
>>>>> In our initial POC using Xen 4.8.x we where using Linux coretemp
>>>>> driver
>>>>> reading by example /class/sys/hwmon/hwmon0/temp3_input but it got
>>>>> deprecated at commit 72e038450d3d5de1a39f0cfa2d2b0f9b3d43c6c6
>>>>
>>>> Hmm, I wouldn't call this deprecation, but a regression. I would
>>>> say we want to re-expose this leaf to Dom0, the more that the
>>>> commit also only mentions unprivileged domains. Andrew?
>>>
>>> AFAICT from the documents provided by Fredy the temperature is read
>>> from a MSR that reports the current temperature of the core on which
>>> the MSR is read from. When running on Xen this will only work
>>> correctly if dom0 is given the same vCPUs as pCPUs and those are
>>> identity pinned.
>>
>> I just want to be sure I got it correctly, by saying "When running on
>> Xen this will only work correctly if ..." means in a future
>> implementation or that right now could work if I pin this v/pCPUS?
> 
> No, right now there's no way to get this data from dom0, regardless of
> the pinning.

Of course you can, using the MSR "device" Linux optionally
provides (plus perhaps the rdmsr utility from the msr-tools
package).

> The commit you mention simply removes exposing the feature on CPUID,
> but I'm not sure whether access to the actual MSR is also forbidden. I
> think so since we do MSR white listing IIRC, and I don't seem to find
> MSR_IA32_THERM_STATUS white listed anywhere.

At least for PV we continue to let everything not specially
handled shine through - see the bottom of pv/emul-pro-op.c:read_msr().

Jan
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