[libvirt] [PATCH 0/2] Avoid issues due to qemu dropping osxsave and ospke

[Daniel Henrique Barboza]

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

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