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

["Fredy P." <fredy.pulido@xxxxxxxxxxxxxxxxxxxx>]

-- 
Fredy Pulido,
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On Thu, 2019-07-25 at 16:07 +0200, Roger Pau Monné wrote:
> On Thu, Jul 25, 2019 at 01:43:34PM +0000, Jan Beulich wrote:
> > 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).
> 
> Right, there's also the fact that all pCPUs should be reported in the
> thermal driver, while dom0 might have less vCPUs than pCPUs on the
> system.
> 
> Do you think you can take a look into this Fredy?

I think I can but will takes long, short history I'm "just a sysadmin
doing a security update" then I'll try to find if some one of the guys
that wrote low level code to help me with this.

> It will involve looking into the Linux driver in order to make use of
> an hypercall instead of a rdmsr. I think it should be fine to expose
> the CPUID leaf to dom0 as long as reads are performed from the
> hypercall, in order to assure that Linux gets consistent values.
> 
> Roger.



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