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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Nov 21, 2019 at 07:09:31PM +0530, 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.
>
> From the patch description
> https://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=72e038450d3d5de1a39f0cfa2d2b0f9b3d43c6c6
> it seems that the change was introduced to not expose EAX(0x06h) to
> unprivileged PV guests but nothing is said for Dom0 itself. I think
> you already mentioned that the flag is hid from Dom0 as well
> intentionally.
> 
> So unless hypervisor wants to do thermal management of the CPU board,
> it would inhibit Dom0's ability to do this function.

That's likely what you want, on a Xen system dom0 is a special guest,
but still a guest, so it's not feasible for a native dom0 driver to do
power or temperature management without having Xen specific
knowledge. For instance the load on dom0 doesn't match the actual
load on the hardware.

I think we had a very similar discussion at:

https://marc.info/?l=xen-devel&m=156397696413230&w=2

I would recommend reading the full thread and the
conclusions/proposals.

Roger.

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