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

[Tim Gardner]

On 8/30/23 8:45 AM, Stefan Bader wrote:
> 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.
> 
> 
Acked-by: Tim Gardner <tim.gardner at canonical.com>
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Tim Gardner
Canonical, Inc