From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:60912) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TOYhP-0006qG-2u for qemu-devel@nongnu.org; Wed, 17 Oct 2012 14:50:36 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1TOYhN-0005Xm-VS for qemu-devel@nongnu.org; Wed, 17 Oct 2012 14:50:35 -0400 Received: from david.siemens.de ([192.35.17.14]:19432) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TOYUY-00027H-TE for qemu-devel@nongnu.org; Wed, 17 Oct 2012 14:37:19 -0400 Message-ID: <507EFADA.7080700@siemens.com> Date: Wed, 17 Oct 2012 20:37:14 +0200 From: Jan Kiszka MIME-Version: 1.0 References: <507EE07E.20504@redhat.com> In-Reply-To: <507EE07E.20504@redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] Disabling KVM "on the fly" List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: Clemens Kolbitsch , qemu-devel@nongnu.org On 2012-10-17 18:44, Paolo Bonzini wrote: > Il 17/10/2012 18:37, Clemens Kolbitsch ha scritto: >> Guys, >> >> I know this is question might seem a bit odd, but I'm curious: >> >> Has anyone ever tried to write code to disable KVM on the fly / is it >> at all possible? I have a situation where I need to use TCG for >> certain parts of the code, but would love to have acceleration for >> everything else. My idea was to pause the VM, then use the >> snapshotting mechanism to dump the state, and then to resume the >> snapshot, but writing the KVM state into the non-KVM structures. > > As a start, you can try using "migrate exec:cat>foo.save" with a KVM > machine and "-incoming 'exec:cat foo.save'" with a TCG machine. The > main problem should be that TCG doesn't implement kvmclock. > > If you disable the KVM interrupt controller and timer (which is just an > implementation detail, not a hardware difference), Unnecessary. Both models (KVM in-kernel and QEMU userspace) are compatible - in the absence of bugs. > the differences > between KVM and TCG are just that KVM doesn't initialize some TCG-only > data structure, and that KVM uses many CPU threads; TCG uses one that > goes through CPUs round-robin. The CPU threads of course execute > different code. > > So no, in theory there is nothing that prevents this from working in > principle, except for kvmclock. -cpu qemu64,-kvmclock should solve that. You also need -global pc-sysfw.rom_only=1 as KVM does not support write protected memory areas and creates an "old-style" BIOS region. But loading a KVM image into TCG lets non-trival guests lock up. Likely due to differences in the CPU virtualization/emulation (MSRs...). Also, certain KVM specific CPU states cannot be easily translated into TCG (and are definitely just ignored in TCG so far). Jan -- Siemens AG, Corporate Technology, CT RTC ITP SDP-DE Corporate Competence Center Embedded Linux