On 09/22/2014 06:31 AM, Christopher Covington wrote:
> On 09/19/2014 05:46 PM, H. Peter Anvin wrote:
>> On 09/19/2014 01:46 PM, Andy Lutomirski wrote:
>>>>
>>>> However, it sounds to me that at least for KVM, it is very easy just to
>>>> emulate the RDRAND instruction. The hypervisor would report to the guest
>>>> that RDRAND is supported in CPUID and the emulate the instruction when
>>>> guest executes it. KVM already traps guest #UD (which would occur if
>>>> RDRAND executed while it is not supported) - so this scheme wouldn’t
>>>> introduce additional overhead over RDMSR.
>>>
>>> Because then guest user code will think that rdrand is there and will
>>> try to use it, resulting in abysmal performance.
>>>
>>
>> Yes, the presence of RDRAND implies a cheap and inexhaustible entropy
>> source.
>
> A guest kernel couldn't make it look like RDRAND is not present to guest
> userspace?
>
It could, but how would you enumerate that? A new "RDRAND-CPL-0" CPUID
bit pretty much would be required.
-hpa
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