Re: Standardizing an MSR or other hypercall to get an RNG seed?

Andy Lutomirski
2014-09-18T15:44:29Z

On Thu, Sep 18, 2014 at 8:38 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Thu, Sep 18, 2014 at 7:43 AM, H. Peter Anvin <hpa@zytor.com> wrote:
>> On 09/18/2014 07:40 AM, KY Srinivasan wrote:
>>>>
>>>> The main questions are what MSR index to use and how to detect the
>>>> presence of the MSR. I've played with two approaches:
>>>>
>>>> 1. Use CPUID to detect the presence of this feature. This is very easy for
>>>> KVM to implement by using a KVM-specific CPUID feature. The problem is
>>>> that this will necessarily be KVM-specific, as the guest must first probe for
>>>> KVM and then probe for the KVM feature. I doubt that Hyper-V, for
>>>> example, wants to claim to be KVM. If we could standardize a non-
>>>> hypervisor-specific CPUID feature, then this problem would go away.
>>>
>>> We would prefer a CPUID feature bit to detect this feature.
>>>
>>
>> I guess if we're introducing the concept of pan-OS MSRs we could also
>> have pan-OS CPUID. The real issue is to get a single non-conflicting
>> standard.
>
> Agreed.
>
> KVM currently puts 0 in 0x40000000.EAX, meaning that a feature bit in
> Microsoft's leaf 0x40000003 would probably not work well for KVM. I
> don't expect that Microsoft wants to start claiming to be KVM for the
> purpose of using a KVM-style feature bit, so, if we went the CPUID
> route, we would probably need something new.

Slight correction: QEMU/KVM has optional support for Hyper-V feature
enumeration. Ideally the RNG seed mechanism would be enabled by
default, but I don't know whether the QEMU maintainers would be okay
with enabling the Hyper-V cpuid mechanism in a default configuration.

--Andy

>
> --Andy
>
>>
>> -hpa
>>
>>
>
>
>
> --
> Andy Lutomirski
> AMA Capital Management, LLC

--
Andy Lutomirski
AMA Capital Management, LLC