Messages in this thread | | | Date | Wed, 16 Jul 2014 09:03:02 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH 0/4] random,x86,kvm: Add and use MSR_KVM_GET_RNG_SEED |
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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
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