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

From: Eduardo Habkost <ehabkost@redhat.com>
To: "Collin L. Walling" <walling@linux.vnet.ibm.com>,
	jjherne@linux.vnet.ibm.com, borntraeger@de.ibm.com,
	libvir-list@redhat.com, Markus Armbruster <armbru@redhat.com>,
	qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [libvirt] [PATCH v1] qemu: command: rework cpu feature argument support
Date: Wed, 16 Nov 2016 12:05:37 -0200	[thread overview]
Message-ID: <20161116140537.GO5057@thinpad.lan.raisama.net> (raw)
In-Reply-To: <20161116131502.GB2238061@orkuz.home>

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@linux.vnet.ibm.com>
> > > > 
> > > > 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.

-- 
Eduardo