linux-kernel - Re: random: Providing a seed value to VM guests

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Message-ID: <068139e3-50f8-4d44-972a-0296b6bc36f7@email.android.com>
Date:	Thu, 01 May 2014 12:40:35 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	tytso@....edu, Andy Lutomirski <luto@...capital.net>
CC:	Florian Weimer <fweimer@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kees Cook <kees@...flux.net>, kvm list <kvm@...r.kernel.org>
Subject: Re: random: Providing a seed value to VM guests

As I said... I think KVM has already added an emulated instructions enumeration API.

On May 1, 2014 12:26:18 PM PDT, tytso@....edu 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?
>
>						- Ted

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