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

On Wed, Nov 16, 2016 at 02:15:02PM +0100, Jiri Denemark wrote: > 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? Not a problem, as we have no plans to remove +feat/-feat support in x86 anymore.