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

["H. Peter Anvin" <hpa@zytor.com>]

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