Re: [Xen-devel] Xen hiding thermal capabilities from Dom0

[Rishi <2rushikeshj@xxxxxxxxx>]

On Thu, Nov 21, 2019 at 7:54 PM Jürgen Groß <jgross@xxxxxxxx> wrote:
>
> On 21.11.19 14:39, Rishi wrote:
> > On Tue, Nov 19, 2019 at 2:47 PM Jan Beulich <jbeulich@xxxxxxxx> wrote:
> >>
> >> On 19.11.2019 06:23, Rishi wrote:
> >>> ok, thanks for clearing it up. Would a patch be accepted if this
> >>> option of showing EAX leaf is selectively done through command line
> >>> (default disabled)?
> >>
> >> In general I'd expect this to be rather unlikely, but I guess much
> >> would depend on the actual reasoning done in the description.
> >>
> >>> On longer run, what is an expected sane model of virtualizing this?
> >>> With some guidance, may be I or someone else can code to bring the
> >>> functionality back.
> >>
> >> Which functionality? So far you've talked of only CPUID bits I
> >> think, without explaining at all what functionality you want to
> >> have that depends on these. In general, as said earlier, CPU
> >> management is the hypervisor's responsibility, so I'd rather
> >> not see this virtualized, but the hypervisor be put into a
> >> position of doing whatever is needed.
> >>
> >> Jan
> >
> > The reasoning to have EAX(0x06h) exposed to Dom0 is for Thermal and
> > Power management.
> > Without EAX(0x06h) Dom0 is unable to sense presence of CPU core
> > temperature or do Thermal management - including but not limited to
> > operating Fan speed.
> > Dom0 has to rely on other possible ways such as ipmi or BIOS which are
> > optionally available.
>
> You are aware that dom0 can't easily control on which _physical_ cpu it
> is just running? So it could easily be that you are sampling lets say
> 3 MSRs in a function, but you are accessing different cpus each time due
> to the hypervisor re-scheduling the vcpu in between.
>
> And in case you want to adjust settings you can hit another cpu again.
>
> So: no, just giving dom0 access to the management hardware isn't going
> to fly. You need to have a proper virtualization layer for that purpose.
>
>
> Juergen

I now understand it, thanks.
As mentioned in other reply, I think a pv version of coretemp Linux
driver may be a way to bring back thermal information values for a
given pCPU.

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