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

[Jürgen Groß <jgross@xxxxxxxx>]

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

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