Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759989AbaGPQDY (ORCPT ); Wed, 16 Jul 2014 12:03:24 -0400 Received: from terminus.zytor.com ([198.137.202.10]:41403 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759734AbaGPQDU (ORCPT ); Wed, 16 Jul 2014 12:03:20 -0400 Message-ID: <53C6A236.7040904@zytor.com> Date: Wed, 16 Jul 2014 09:03:02 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: Andy Lutomirski , Paolo Bonzini CC: Daniel Borkmann , Gleb Natapov , kvm list , "Theodore Ts'o" , "linux-kernel@vger.kernel.org" , Kees Cook , X86 ML , Srivatsa Vaddagiri , Raghavendra K T Subject: Re: [PATCH 0/4] random,x86,kvm: Add and use MSR_KVM_GET_RNG_SEED References: <20140716064110.GV18167@minantech.com> <53C62563.6050106@redhat.com> <53C62B68.50702@redhat.com> In-Reply-To: X-Enigmail-Version: 1.6 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/