Re: [PATCH 10/16] x86/cpu: Detect VMX features on Intel, Centaur and Zhaoxin CPUs

[Paolo Bonzini]

On 07/10/19 21:54, Sean Christopherson wrote:
>> For QEMU, we're defining a feature as supported if a feature can be
>> turned both on and off.  Since msr_low and msr_high can be defined
>> respectively as must-be-one and can-be-one, the features become
>> "msr_high & ~msr_low".
> 
> That makes sense for Qemu, but I don't think it's appropriate for this
> type of reporting.  E.g. if EPT and Unrestricted Guest are must-be-one on
> a hypothetical (virtual) CPU, it'd be odd to not list them as a supported
> feature.
> 
> For actual hardware (well, Intel hardware), as proposed it's a moot point.
> The only features that are must-be-one (even without "true" MSRs) and are
> documented in the SDM are CR3_LOAD_EXITING, CR3_STORE_EXITING,
> SAVE_DEBUG_CONTROLS, and LOAD_DEBUG_CONTROLS, none of which are reported
> in /proc/cpuinfo.
> 
>> Also, shouldn't this use the "true" feature availability MSRs if available?
> 
> Only if incorporating the "& ~msr_low" can-be-one logic.  If a feature is
> considered supported if it must-be-one or can-be-one then the true MSR and
> vanilla MSR will yield the same feature set.

Ok, that all makes sense.

Paolo