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

[Jan Beulich <jbeulich@xxxxxxxx>]

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

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