This is a VM with 'osxsave' capability declared, when there is no
'osxsave' guest support, using this series on top of current master:

$ sudo ./run tools/virsh start ub1810-osxsave
error: Failed to start domain ub1810-osxsave
error: internal error: process exited while connecting to monitor: 2019-05-09T17:10:41.077568Z qemu-system-x86_64: can't apply global Skylake-Client-IBRS-x86_64-cpu.osxsave=off: Property '.osxsave' not found

Applying this series makes the VM boot up, as intended.

I believe there is room for discussion whether the VM should boot
up or fail to boot with a "feature not found" error like the case below,
in a sense of "you can't disable a feature that does not exist":

<feature policy='disable' name='not_a_cap'/>

$ sudo ./run tools/virsh start ub1810-unknown-cap
error: Failed to start domain ub1810-unknown-cap
error: unsupported configuration: unknown CPU feature: not_a_cap

But the alternative presented in this series does the trick and it's
less annoying for existing VMs.

For all the patch series:

Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Tested-by: Daniel Henrique Barboza <danielhb413@gmail.com>

On 4/25/19 9:50 AM, Christian Ehrhardt wrote:

Hi,
this series tries to address a drop of commandline options by qemu in regard to
osxsave [1] and ospke [2].
This was already discussed in [3] late last year but got forgotten afterwards.
The Ubuntu bug is at [4] and an older Fedora bug is at [5].

TL;DR:
- osxsave/ospke features were never really configurable
- KVM never returned the bits on GET_SUPPORTED_CPUID
- very rare to be seen in the wild
- avoid issues with newer qemu and old/odd XMLs to be sure

Details:

I checked various use cases from virt-install to openstack and some in between.
The only cases I found that would define osxsave/ospke is virt-install pior
to version 2.0 and even there only when used with --cpu=host-model or
--cpu=host-copy.
If you ever really enabled the feature you'd have got:
  error: the CPU is incompatible with host CPU:
  Host CPU does not provide required features: ospke

The problem lies in domain XMLs that explicitly disable it. That would be
    <feature policy='disable' name='osxsave'/>
But due to almost (or actually none) no host exposing this the following
also triggers:
    <feature policy='optional' name='ospke'/>

This will make libvirt add it to the qemu commandline like:
  -cpu ...,osxsave=off,ospke=off

And that will crash when qemu starts with:
  error: internal error: process exited while connecting to monitor:
  2019-04-25T12:12:01.698646Z qemu-system-x86_64: can't apply global
  core2duo-x86_64-cpu.osxsave=off: Property '.osxsave' not found

There are much more long term discussions about demoting and dropping qemu
features and I'd like to avoid those discussions being mixed.
The reason to drop it more or less without notice was that it never did
anything to begin with. Due to that our solution might in a similar fashion
be more trivial - just stop defining those two features to qemu commandline.

[1]: https://git.qemu.org/?p=qemu.git;a=commit;h=f1a23522b03a569f13aad49294bb4c4b1a9500c7
[2]: https://git.qemu.org/?p=qemu.git;a=commit;h=9ccb9784b57804f5c74434ad6ccb66650a015ffc
[3]: https://www.mail-archive.com/qemu-devel@nongnu.org/msg561877.html
[4]: https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1825195
[5]: https://bugzilla.redhat.com/show_bug.cgi?id=1644848

Christian Ehrhardt (2):
  qemu: do not define known no-op features
  qemuxml2argvtest: add test for remove cpu features

 src/qemu/qemu_command.c                       | 23 +++++++++++++++
 .../qemuxml2argvdata/cpu-host-model-cmt.args  |  2 +-
 .../cpu-no-removed-features.args              | 29 +++++++++++++++++++
 .../cpu-no-removed-features.xml               | 23 +++++++++++++++
 tests/qemuxml2argvdata/cpu-tsc-frequency.args |  4 +--
 tests/qemuxml2argvtest.c                      |  1 +
 6 files changed, 79 insertions(+), 3 deletions(-)
 create mode 100644 tests/qemuxml2argvdata/cpu-no-removed-features.args
 create mode 100644 tests/qemuxml2argvdata/cpu-no-removed-features.xml