Re: [PATCH v2] KVM: x86: use CPUID to locate host page table reserved bits

[Paolo Bonzini]

On 11/12/19 01:11, Huang, Kai wrote:
>> kvm_get_shadow_phys_bits() must be conservative in that:
>>
>> 1) if a bit is reserved it _can_ return a value higher than its index
>>
>> 2) if a bit is used by the processor (for physical address or anything
>> else) it _must_ return a value higher than its index.
>>
>> In the SEV case we're not obeying (2), because the function returns 43
>> when the C bit is bit 47.  The patch fixes that.
> Could we guarantee that C-bit is always below bits reported by CPUID?

That's a question for AMD. :)  The C bit can move (and probably will,
otherwise they wouldn't have bothered adding it to CPUID) in future
generations of the processor.

Paolo