Re: [libvirt] [PATCH v1] qemu: command: rework cpu feature argument support
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>
>
> 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.
--
Eduardo