Re: [Qemu-devel] [PATCH 1/1] x86: add CPU flags supported inside libvirt

[Jiri Denemark]

On Thu, Jul 18, 2019 at 16:45:37 +0300, Denis V. Lunev wrote:
> There are the following flags available in libvirt inside cpu_map.xm
>     <feature name='cvt16'>
>       <cpuid function='0x80000001' ecx='0x00040000'/>
>     </feature>
>     <feature name='cmt'> <!-- cqm -->
>       <cpuid eax_in='0x07' ecx_in='0x00' ebx='0x00001000'/>
>     </feature>
> We have faced the problem that QEMU does not start once these flags are
> present in the domain.xml.

Libvirt should not add this to the XML by itself (when using host-model
CPU, for example) so the user must have asked for the feature
explicitly. Thus I don't see any problem with QEMU refusing to start
with such configuration. And the workaround is easy, just don't do it.

I'm not sure about cvt16, but IIRC cmt and mbm_* features were added as
a way to detect whether the host CPU supports perf monitoring counters.
I think tt was not the brightest idea, but there's no reason why QEMU
should support enabling these features. Unless it actually makes sense
for QEMU.

If there are any issues with libvirt passing these features to QEMU
without explicit request from the user, we should address them in
libvirt.

Jirka