From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:52827) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TCShd-00067l-C2 for qemu-devel@nongnu.org; Fri, 14 Sep 2012 06:00:53 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1TCShX-0008S2-Hg for qemu-devel@nongnu.org; Fri, 14 Sep 2012 06:00:49 -0400 Received: from goliath.siemens.de ([192.35.17.28]:30906) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TCShX-0008Rs-6D for qemu-devel@nongnu.org; Fri, 14 Sep 2012 06:00:43 -0400 Message-ID: <50530047.9030703@siemens.com> Date: Fri, 14 Sep 2012 12:00:39 +0200 From: Jan Kiszka MIME-Version: 1.0 References: <5052DF3A.1070007@msgid.tls.msk.ru> <20120914093304.GC6819@redhat.com> <5052FB60.3060109@msgid.tls.msk.ru> In-Reply-To: <5052FB60.3060109@msgid.tls.msk.ru> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] qemu-system-i386 vs qemu-system-x86_64 ? List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Michael Tokarev Cc: qemu-devel On 2012-09-14 11:39, Michael Tokarev wrote: > On 14.09.2012 13:33, Daniel P. Berrange wrote: >> On Fri, Sep 14, 2012 at 11:39:38AM +0400, Michael Tokarev wrote: >>> What's the difference between the two except that the >>> latter adds some more instructions (actually whole new >>> subsytem) to the former? Why do we need -i386, what >>> -x86_64 does not do which does -i386? >> >> AFAIK, qemu-system-x86_64 does not support KVM on a 32-bit host. > > Isn't it the other way around? > > Or, does qemu-system-i386 EVER support kvm? It does support KVM on 32-bit (minus undiscovered bugs in 32-bit-only kernel code paths once in a while - none known right now, though). Don't confuse old qemu-kvm with QEMU here. The latter always supported KVM with both variants, and qemu-kvm now also does as it stopped being different. The major difference in qemu-system-i386 vs. qemu-system-x86_64 is on the TCG side: We measured noticeable performance benefits when running 32/16 bit OSes against qemu-system-i386 vs. using qemu-system-x86_64. I don't have numbers at hand, but colleagues decided to use the 32-bit version for that reason (when no KVM is available). Jan -- Siemens AG, Corporate Technology, CT RTC ITP SDP-DE Corporate Competence Center Embedded Linux