linux-kernel - Re: [PATCH 0/4] random,x86,kvm: Add and use MSR_KVM_GET_RNG

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Date:	Wed, 16 Jul 2014 09:03:02 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Paolo Bonzini <pbonzini@...hat.com>
CC:	Daniel Borkmann <dborkman@...hat.com>,
	Gleb Natapov <gleb@...nel.org>, kvm list <kvm@...r.kernel.org>,
	"Theodore Ts'o" <tytso@....edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Subject: Re: [PATCH 0/4] random,x86,kvm: Add and use MSR_KVM_GET_RNG_SEED

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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