Re: [libvirt] [Qemu-devel] [PATCH v1] qemu: command: rework cpu feature argument support

CCing qemu-devel. CCing Markus, in case he has any insights about the interface introspection. On Tue, Nov 15, 2016 at 08:42:12AM +0100, Jiri Denemark wrote: > On Mon, Nov 14, 2016 at 18:02:29 -0200, Eduardo Habkost wrote: > > On Mon, Nov 14, 2016 at 02:26:03PM -0500, Collin L. Walling wrote: > > > cpu features are passed to the qemu command with feature=on/off > > > instead of +/-feature. > > > > > > Signed-off-by: Collin L. Walling <walling(a)linux.vnet.ibm.com&gt; > > > > If I'm not mistaken, the "feature=on|off" syntax was added on > > QEMU 2.0.0. Does current libvirt support older QEMU versions? > > Of course it does. I'd love to switch to feature=on|off, but how can we > check if QEMU supports it? We can't really start using this syntax > without it. Actually, I was wrong, this was added in v2.4.0. "feat=on|off" needs two things to work (in x86): * Translation of all "foo=bar" options to QOM property setting. This was added in v2.0.0-rc0~162^2 * The actual QOM properties for feature names to be present. They were added in v2.4.0-rc0~101^2~1 So you can be sure "feat=on" is supported by checking if the feature flags are present in device-list-properties output for the CPU model. But device-list-properties is also messy[1]. Maybe we can use the availability of query-cpu-model-expansion to check if we can safely use the new "feat=on|off" system? It's easier than taking all the variables above into account. --- [1] * device-list-properties support for x86 CPU QOM classes will be in QEMU 2.8. * device-list-properties on x86 CPU QOM classes returns an error on QEMU 2.5-2.7. * device-list-properties on x86 CPU classes may crash QEMU in QEMU older than 2.5 (see commit 4c315c27). But: * query-cpu-definitions will probably return the CPU QOM typename in QEMU 2.9+ only.