[SRU Jammy PATCH 0/1] Change TDP_MMU default for 5.15 kernels

[Stefan Bader]

Impact:
We had reports of VM setups which would show intermediate crashes and after
that locking up completely. This could be reproduced with large memory setups.
The problem seems to be that fixes to performance regressions caused more
problems in 5.15 kernels and the full fixes are too intrusive to be backported.

Fix:
The following patch was recently sent to the upstream stable mailing list and
looks to be making its way into linux-5.15.y. This changes the default value
of kvm.tdp_mmu to off (if anyone is willing to take the risks, this can be
changed back in config).

Regression potential:
VM hosts with many large memory tennants might see a performance impact which
the TDP MMU approach tried to solve. If those did not see other problems they
might turn this on again.

Testcase:
Large openstack instance (64GB memory, AMD CPU (using SVM)) with a large second
level guest (32GB memory). Repeatedly starting and stopping the 2nd level guest.