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

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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@xxxxxxxxxxxxxxxxxx>
> > > 
> > > 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?

Jirka

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