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

H. Peter Anvin
2014-07-16T16:03:02Z

On 07/16/2014 07:07 AM, Andy Lutomirski wrote:
>
> This patch has nothing whatsoever to do with how much I trust the CPU
> vs the hypervisor. It's for the enormous installed base of machines
> without RDRAND.
>
> hpa suggested emulating RDRAND awhile ago, but I think that'll
> unusably slow -- the kernel uses RDRAND in various places where it's
> expected to be fast, and not using it at all will be preferable to
> causing a VM exit for every few bytes. I've been careful to only use
> this in the guest in places where a few hundred to a few thousand
> cycles per 64 bits of RNG seed is acceptable.
>

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."

-hpa