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

On Tue, Nov 15, 2016 at 11:44:00 -0200, Eduardo Habkost wrote: > 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. Yeah, this could work since s390 already supports query-cpu-model-expansion. It would cause feature=on|off not to be used on x86_64 with QEMU older than 2.9.0, but I guess that's not a big deal, is it?