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

[Rishi <2rushikeshj@xxxxxxxxx>]

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.

What is an alternative way for coretemp kernel module to detect
"DTHERM" processor flag and/or proceed for safe reading of MSR to do
further temperature value reads?

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