Re: [PATCH 1/7] hyperv: Detect Nested virtualization support for SVM

[Vineeth Pillai]

Hi Vitaly,

On 4/8/21 7:06 AM, Vitaly Kuznetsov wrote:
> -	if (ms_hyperv.hints & HV_X64_ENLIGHTENED_VMCS_RECOMMENDED) {
> +	/*
> +	 * AMD does not need enlightened VMCS as VMCB is already a
> +	 * datastructure in memory.
> Well, VMCS is also a structure in memory, isn't it? It's just that we
> don't have a 'clean field' concept for it and we can't use normal memory
> accesses.

Yes, you are right. I was referring to the fact that we cant use normal
memory accesses, but is a bit mis-worded.

> > 	We need to get the nested
> > +	 * features if SVM is enabled.
> > +	 */
> > +	if (boot_cpu_has(X86_FEATURE_SVM) ||
> > +	    ms_hyperv.hints & HV_X64_ENLIGHTENED_VMCS_RECOMMENDED) {
> Do I understand correctly that we can just look at CPUID.0x40000000.EAX
> and in case it is >= 0x4000000A we can read HYPERV_CPUID_NESTED_FEATURES
> leaf? I'd suggest we do that intead then.
I agree, that is a better way to consolidate both the cases.
Will change it in the next iteration. Probably the above code
comment is not needed when we consolidate the cases here.

Thanks,
Vineeth