From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:54526) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TIyyw-0001UB-GM for qemu-devel@nongnu.org; Tue, 02 Oct 2012 05:41:44 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1TIyyr-0002nn-PY for qemu-devel@nongnu.org; Tue, 02 Oct 2012 05:41:38 -0400 Received: from hall.aurel32.net ([88.191.126.93]:57899) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TIyyr-0002mh-Ij for qemu-devel@nongnu.org; Tue, 02 Oct 2012 05:41:33 -0400 Date: Tue, 2 Oct 2012 10:15:02 +0200 From: Aurelien Jarno Message-ID: <20121002081502.GI4623@ohm.aurel32.net> References: <50694EC1.8060006@siemens.com> <20121001093102.GA14797@amt.cnet> <50696E9E.7030302@siemens.com> <87zk468h3y.fsf@codemonkey.ws> <50699CDC.1010604@redhat.com> <87wqzap9ui.fsf@codemonkey.ws> <5069A9DF.4040606@siemens.com> <5069C814.7010201@suse.de> <20121001164730.GC15554@redhat.com> <87zk4571vf.fsf@blackfin.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <87zk4571vf.fsf@blackfin.pond.sub.org> Subject: Re: [Qemu-devel] [PATCH] kvm: Set default accelerator to "kvm" if the host supports it List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Markus Armbruster Cc: kvm , Scott Moser , Jan Kiszka , Marcelo Tosatti , Michael Tokarev , qemu-devel , Bruce Rogers , Avi Kivity , Anthony Liguori , Cole Robinson , Paolo Bonzini , Andreas =?iso-8859-15?Q?F=E4rber?= On Tue, Oct 02, 2012 at 09:46:12AM +0200, Markus Armbruster wrote: > "Daniel P. Berrange" writes: > > > On Mon, Oct 01, 2012 at 06:43:00PM +0200, Andreas Färber wrote: > >> Hello Jan, > >> > >> Am 01.10.2012 16:34, schrieb Jan Kiszka: > >> > If we built a target for a host that supports KVM in principle, set the > >> > default accelerator to KVM as well. This also means the start of QEMU > >> > will fail to start if KVM support turns out to be unavailable at > >> > runtime. > >> > >> From a distro point of view this of course means that we will build > >> against KVM and that the new KVM default will start to fail for users on > >> very old hardware. Can't we do a runtime check to select the default? > > > > NB, this is *not* only about old hardware. There are plenty of users who > > use QEMU inside VMs. One very common usage I know of is image building > > tools which are run inside Amazon VMs, using libguestfs & QEMU. > > > > IMHO, default to KVM, fallback to TCG is the most friendly default > > behaviour. > > Friendly perhaps, generating an infinite series of questions "why is my > guest slow as molasses?" certainly. > > And for each instance of the question, there's an unknown number of > users who give QEMU a quick try, screw up KVM unknowingly, observe the > glacial speed, and conclude it's crap. > That's why it should not fallback silently to TCG, but warn the user about that. On the other hand, on a machine without KVM support (which might just be because of local policy admin policy which doesn't provide access the /dev/kvm, nested virtualization, etc.), if QEMU fails to start (while previous versions were working), the user can conclude that QEMU is crap. -- Aurelien Jarno GPG: 1024D/F1BCDB73 aurelien@aurel32.net http://www.aurel32.net