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
