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

[Jan Beulich <JBeulich@xxxxxxxx>]

On 25.07.2019 15:13, 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.
> 
> Not sure how common this MSR interface is in order to read thermal
> values, if the interface it's common maybe it's something that could
> be implemented in Xen, and exported somehow to dom0, maybe using
> sysctl?
> 
> Or else having an hypercall that allows dom0 to request Xen to execute
> MSR read/writes on a given pCPU.

This would look to require just a small extension to
XEN_RESOURCE_OP_MSR_READ. Question is whether the Linux driver
maintainers would accept a change using this Xen-specific
alternative access mechanism (in whatever shape).

Jan
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