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

[Rishi <2rushikeshj@xxxxxxxxx>]

On Thu, Nov 14, 2019 at 10:05 PM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> On 14.11.2019 17:07, Rishi wrote:
> > In some of our hosts, Xen is not correctly exposing processor thermal
> > capabilities to Dom0.
> > Please refer: https://bugzilla.kernel.org/show_bug.cgi?id=205347
> >
> > The flag
> > /* Thermal and Power Management Leaf, CPUID level 0x00000006 (EAX), word 14 
> > */
> > X86_FEATURE_DTHERM (14*32+ 0)
> >
> > is returned 0 via PVOP_VCALL4 vs returns 1 via native_cpuid() call from 
> > Dom0.
> >
> > Sample output via PVCALL vs Native call.
> > [63291.688755] cpuid_eax 6 :  a 0
> > [63291.688759] native_cpuid : a 77
> >
> > Is this a bug or needs some special feature to be enabled from Xen command 
> > line?
>
> Exposing this to guests (including Dom0) would imply properly
> virtualizing the respective behavior. In
> xen/arch/x86/cpuid.c:recalculate_misc() we specifically have
>
>     p->basic.raw[0x6] = EMPTY_LEAF; /* Therm/Power not exposed to guests. */
>
> If you wanted this exposed, you'd first of all need to come up
> with a sane virtualization model of this functionality. Simply
> exposing the CPUID bits is not going to be an option.
>
> Jan

Is it due to any security risks? If so, are there any advisories around it?

Should it be allowed that Xen is hiding CPU flags (in this case
Therm), yet a modification to Dom0 kernel allows them to be brought
back?

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