Global option to configure tb-cache size for tcg feature

[Yatin Karel]

Hi Team,

In OpenStack Upstream CI as part of testing we have to create multiple
guest vms together on a 8GB VM(no nested-virt support) so we have to use
qemu emulation. But with Qemu>=5 'tb-cache' size has increased from 32MiB
to 1GiB and due to this we cannot run multiple guest vms together as we
have limited resources in CI. We tried to use some swap together but that
too didn't helped much as it get's too slow.

Thanks Libvirt-8.0.0 now allows to configure tcg feature as below in
instance domain.xml:-
  <features>
    <tcg>
      <tb-cache unit='KiB'>32768</tb-cache>
    </tcg>
  </features>

Is there some global option available in libvirt/qemu which we can use
instead of changing each and every xml so we can avoid code changes in nova
code[1] for just the CI use case?

[1] https://bugs.launchpad.net/nova/+bug/1949606

PS: Please keep me in CC for responses as I am not subscribed to the
mailing list.

Thanks and Regards
Yatin Karel
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