From mboxrd@z Thu Jan 1 00:00:00 1970 From: Markus Armbruster Subject: Re: [PATCH] kvm: Set default accelerator to "kvm" if the host supports it Date: Tue, 02 Oct 2012 09:46:12 +0200 Message-ID: <87zk4571vf.fsf@blackfin.pond.sub.org> References: <20120930191146.GA20012@amt.cnet> <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> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Cc: kvm , Scott Moser , Jan Kiszka , Marcelo Tosatti , Michael Tokarev , qemu-devel , Bruce Rogers , Avi Kivity , Anthony Liguori , Cole Robinson , Paolo Bonzini , Andreas =?utf-8?Q?F=C3=A4rber?= To: "Daniel P. Berrange" Return-path: In-Reply-To: <20121001164730.GC15554@redhat.com> (Daniel P. Berrange's message of "Mon, 1 Oct 2012 17:47:30 +0100") List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+gceq-qemu-devel=gmane.org@nongnu.org Sender: qemu-devel-bounces+gceq-qemu-devel=gmane.org@nongnu.org List-Id: kvm.vger.kernel.org "Daniel P. Berrange" writes: > On Mon, Oct 01, 2012 at 06:43:00PM +0200, Andreas F=C3=A4rber wrote: >> Hello Jan, >>=20 >> 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. >>=20 >> 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. From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:50923) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TIxBM-0006L5-7r for qemu-devel@nongnu.org; Tue, 02 Oct 2012 03:46:21 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1TIxBL-0005FC-Ca for qemu-devel@nongnu.org; Tue, 02 Oct 2012 03:46:20 -0400 Received: from mx1.redhat.com ([209.132.183.28]:53205) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TIxBL-0005F6-4C for qemu-devel@nongnu.org; Tue, 02 Oct 2012 03:46:19 -0400 From: Markus Armbruster References: <20120930191146.GA20012@amt.cnet> <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> Date: Tue, 02 Oct 2012 09:46:12 +0200 In-Reply-To: <20121001164730.GC15554@redhat.com> (Daniel P. Berrange's message of "Mon, 1 Oct 2012 17:47:30 +0100") Message-ID: <87zk4571vf.fsf@blackfin.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable 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: "Daniel P. Berrange" Cc: kvm , Scott Moser , Jan Kiszka , Marcelo Tosatti , Michael Tokarev , qemu-devel , Bruce Rogers , Avi Kivity , Anthony Liguori , Cole Robinson , Paolo Bonzini , Andreas =?utf-8?Q?F=C3=A4rber?= "Daniel P. Berrange" writes: > On Mon, Oct 01, 2012 at 06:43:00PM +0200, Andreas F=C3=A4rber wrote: >> Hello Jan, >>=20 >> 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. >>=20 >> 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.