From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [RFC PATCH 17/17] KVM: PPC: Add an ioctl for userspace to select which platform to emulate Date: Thu, 30 Jun 2011 11:34:53 +0300 Message-ID: <4E0C352D.60008@redhat.com> References: <20110629101552.GA25406@bloggs.ozlabs.ibm.com> <20110629104103.GR25406@bloggs.ozlabs.ibm.com> <20110629115346.GA17551@zod.rchland.ibm.com> <20110629115857.GB17551@zod.rchland.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Alexander Graf , Paul Mackerras , linuxppc-dev@ozlabs.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org To: Josh Boyer Return-path: Received: from mx1.redhat.com ([209.132.183.28]:61782 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751431Ab1F3IfF (ORCPT ); Thu, 30 Jun 2011 04:35:05 -0400 In-Reply-To: <20110629115857.GB17551@zod.rchland.ibm.com> Sender: kvm-owner@vger.kernel.org List-ID: On 06/29/2011 02:58 PM, Josh Boyer wrote: > >> This makes me wonder if a similar thing might eventually be usable for > >> running an i686 or x32 guest on an x86_64 KVM host. I have no idea if > >> that is even theoretically possible, but if it is it might be better to > >> rename the ioctl to be architecture agnostic. > > > >On x86 this is not required unless we want to "virtualize" pre-CPUID CPUs. Everything as of Pentium has a full bitmap of feature capabilities that KVM gets from user space, including information such as "Can we do 64-bit mode?". > > Ah. Thank you for the explanation. To clarify a bit further, running an i686 guest on an x86_64 host is not only theoretically possible, but is done regularly. First, x86_64 is backwards compatible with i686 (so you can install a 32-bit OS on 64-bit hardware), and second, you can impersonate 32-bit guest hardware on a 64-bit host. -- error compiling committee.c: too many arguments to function