I performed a bisect between the 5.4 and 5.15 kernels. The performance
regression was introduced by a stable update in 5.15.57 by the following
commit:
62b4db57eefec ("x86/entry: Add kernel IBRS implementation")
This commit applies IBRS kernel mitigation for Spectre_v2. IBRS is:
Indirect Branch Restricted Speculation.
This commit was also applied up upstream stable 5.4 with the following SHA1:
a3111faed5c1d ("x86/entry: Add kernel IBRS implementation")
However, the backport to 5.4 did not introduce as much as a performance
regression as the backport to 5.15. There are many difference between
the 5.4 and 5.15 backports of the commit. Much of the assembly logic in
5.15 does not exist in 5.4, since it was not needed.
There are several commits that are later applied to 5.15 stable that
depend on this patch, so it would not be easily reverted.
One option is to use a boot option to disable IBRS mitigation. However,
the security versus performance trade-off must be considered carefully.
IBRS can be disabled with the boot parameter "noibrs"
There is a wiki page that describes the various boot parameters here:
https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/SpectreAndMeltdown/MitigationControls
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https://bugs.launchpad.net/bugs/2042564
Title:
Performance regression in the 5.15 Ubuntu 20.04 kernel compared to 5.4
Ubuntu 20.04 kernel
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