On Thu, Nov 17, 2016 at 04:04:33PM +0100, Markus Armbruster wrote:
Eduardo Habkost <ehabkost@redhat.com> 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 <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.
---
[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 it?
* 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 typename.
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?
That would work for x86, but not for s390x on QEMU <= 2.8 (where only feat=on|off is supported). Using query-cpu-model-expansion as a witness would work for both s390x and i386, even with QEMU <= 2.8. -- Eduardo