From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752180AbaEAUav (ORCPT ); Thu, 1 May 2014 16:30:51 -0400 Received: from terminus.zytor.com ([198.137.202.10]:40365 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751481AbaEAUat (ORCPT ); Thu, 1 May 2014 16:30:49 -0400 User-Agent: K-9 Mail for Android In-Reply-To: References: <20140501192618.GA25829@thunk.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 Subject: Re: random: Providing a seed value to VM guests From: "H. Peter Anvin" Date: Thu, 01 May 2014 13:30:25 -0700 To: Andy Lutomirski , "Ted Ts'o" CC: "linux-kernel@vger.kernel.org" , kvm list , Florian Weimer , Kees Cook Message-ID: <900441a0-24e8-4976-ad35-3971805b9f34@email.android.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org RDSEED is not synchronous. It is, however, nonblocking. On May 1, 2014 1:16:40 PM PDT, Andy Lutomirski wrote: >On May 1, 2014 12:26 PM, wrote: >> >> On Thu, May 01, 2014 at 12:02:49PM -0700, Andy Lutomirski wrote: >> > >> > Is RDSEED really reasonable here? Won't it slow down by several >> > orders of magnitude? >> >> That is I think the biggest problem; RDRAND and RDSEED are fast if >> they are native, but they will involve a VM exit if they need to be >> emulated. So when an OS might want to use RDRAND and RDSEED might be >> quite different if we know they are being emulated. >> >> Using the RDRAND and RDSEED "api" certainly makes sense, at least for >> x86, but I suspect we might want to use a different way of signalling >> that a VM guest can use RDRAND and RDSEED if they are running on a >CPU >> which doesn't provide that kind of access. Maybe a CPUID extended >> function parameter, if one could be allocated for use by a Linux >> hypervisor? >> > >I'm still not convinced. This will affect userspace as well as the >guest kernel, and I don't see why guest user code should be able to >access this API. RDRAND for CPL0 only would work, but that seems odd. > >And I think that RDSEED emulation is asking for trouble. RDSEED is >synchronous, but /dev/random is asynchronous. And making bootup wait >for even a single byte from /dev/random seems bad. In any event, >virtio-rng should be a better interface for this. > >> - Ted >> -- Sent from my mobile phone. Please pardon brevity and lack of formatting.