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

From: Markus Armbruster
Subject: Re: [Qemu-devel] [PATCH v1] qemu: command: rework cpu feature argument support
Date: Thu, 17 Nov 2016 16:04:33 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

Eduardo Habkost <address@hidden> writes:

> 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 <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.
> ---
> [1] * device-list-properties support for x86 CPU QOM classes
>       will be in QEMU 2.8.
>     * device-list-properties on x86 CPU QOM classes returns
>       an error on QEMU 2.5-2.7.
>     * device-list-properties on x86 CPU classes may crash QEMU in
>       QEMU older than 2.5 (see commit 4c315c27).
>   But:
>     * query-cpu-definitions will probably return the CPU QOM
>       typename in QEMU 2.9+ only.

In other words:

* feature=on|off works since 2.4, but what's the best way to probe for

* device-list-properties can act as a witness, but there are two
  problems: you need to map between CPU model and QOM typename to use
  it, and it can fail or even crash before 2.8.

* 2.9 will take provide the information to map between CPU model and QOM

I guess the sane choice is to probe for the CPU model - QOM typename
mapping information, and if it's there, assume device-list-properties
and feature=on|off work.  Eduardo, what do you think?

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