From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:33513) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XMdPa-00080N-Dq for qemu-devel@nongnu.org; Wed, 27 Aug 2014 09:37:24 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1XMdPU-0007OK-93 for qemu-devel@nongnu.org; Wed, 27 Aug 2014 09:37:18 -0400 Received: from mx1.redhat.com ([209.132.183.28]:19737) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1XMdPU-0007OA-1n for qemu-devel@nongnu.org; Wed, 27 Aug 2014 09:37:12 -0400 Message-ID: <53FDDEF3.2000705@redhat.com> Date: Wed, 27 Aug 2014 15:36:51 +0200 From: Paolo Bonzini MIME-Version: 1.0 References: <1408999550-20321-1-git-send-email-ehabkost@redhat.com> <53FC83F5.2090905@redhat.com> <20140826180107.GD32084@thinpad.lan.raisama.net> In-Reply-To: <20140826180107.GD32084@thinpad.lan.raisama.net> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8bit Subject: Re: [Qemu-devel] [PATCH v2 0/6] target-i386: Make most CPU models work with "enforce" out of the box List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Eduardo Habkost Cc: Aurelien Jarno , qemu-devel@nongnu.org, kvm@vger.kernel.org, =?windows-1252?Q?Andreas_F=E4rber?= Il 26/08/2014 20:01, Eduardo Habkost ha scritto: > On Tue, Aug 26, 2014 at 02:56:21PM +0200, Paolo Bonzini wrote: >> Il 25/08/2014 22:45, Eduardo Habkost ha scritto: >>> >>> TCG users expect the default CPU model to contain most TCG-supported features >>> (and it makes sense). See, for example, commit >>> f1e00a9cf326acc1f2386a72525af8859852e1df. >> >> It doesn't though (SMAP is the most egregious omission, and probably the >> main reason why people use QEMU TCG these days), and it raises the >> question of backwards-compatibility of qemu64---should we disable TCG >> features in old machine types? Probably yes, but we've never done that. > > Had we changed qemu64, any changes to the feature set of qemu64 would > probably require compatibility code on old machine-types for KVM, > anyway. But the last time qemu64 was changed was in 2009 (commit > f1e00a9cf326acc1f2386a72525af8859852e1df), it looks like everybody was > afraid of touching "qemu64" because its purpose was not very clear. > > So maybe that's good news, as things can be simpler if we make both TCG > and KVM have similar behavior: > > * qemu64: a conservative default that should work out of the box on > most systems, for both TCG and KVM. That's already the current status, > we just need to document it. > > * -cpu host: for people who want every possible feature to be enabled > (but without cross-version live-migration support). We can easily add > support for "-cpu host" to TCG, too. This means that "-cpu host" has different meanings in KVM and TCG. Is that an advantage or a disadvantage? If I have to choose blindly, I'd rather give different (but sane) meanings to "-cpu qemu64" and the same meanings to "-cpu host"... Basically "-cpu qemu32/64" on KVM would be changed automatically to kvm32/64. Paolo