Re: [PATCH 0/4] random,x86,kvm: Add and use MSR_KVM_GET_RNG_SEED

H. Peter Anvin
2014-07-16T20:21:15Z

On 07/16/2014 09:21 AM, Gleb Natapov wrote:
> On Wed, Jul 16, 2014 at 09:13:23AM -0700, H. Peter Anvin wrote:
>> On 07/16/2014 09:08 AM, Paolo Bonzini wrote:
>>> Il 16/07/2014 18:03, H. Peter Anvin ha scritto:
>>>> I suggested emulating RDRAND *but not set the CPUID bit*. We already
>>>> developed a protocol in KVM/Qemu to enumerate emulated features (created
>>>> for MOVBE as I recall), specifically to service the semantic "feature X
>>>> will work but will be substantially slower than normal."
>>>
>>> But those will set the CPUID bit. There is currently no way for KVM
>>> guests to know if a CPUID bit is real or emulated.
>>>
>>
>> OK, so there wasn't any protocol implemented in the end. I sit corrected.
>>
> That protocol that was implemented is between qemu and kvm, not kvm and a guest.
>

Either which way, the notion was to have a PV CPUID bit like the
proposed kvm_get_rng_seed bit, but to have it exercised by executing RDRAND.

The biggest reason to *not* do this would be that with an MSR it is not
available to guest user space, which may be better under the circumstances.

-hpa