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

From: Collin L. Walling
Subject: Re: [Qemu-devel] [libvirt] [PATCH v1] qemu: command: rework cpu feature argument support
Date: Wed, 16 Nov 2016 10:44:47 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0

On 11/16/2016 09:05 AM, Eduardo Habkost wrote:
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

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 <address@hidden>
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.

Beautiful. Thanks for your responses everyone. :)

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