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

[H. Peter Anvin]

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


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/