[SRU][Jammy][PATCH 0/1] Add split lock detection for EMR

[Roxana Nicolescu]

BugLink: https://bugs.launchpad.net/bugs/2015855

SRU Justification

[Impact]
Intel has introduced support for their new Emerald Rapids CPU.
It was backported to Jammy in #lp2015372

The remaining feature needed to fully support EMR is the split lock
detection mechanism.
So far, bit 5 in IA32_CORE_CAPABILITIES says whether split lock
detection is supported or not, but this is not architectural, meaning
that this should be trusted only if it’s confirmed that a specific CPU
model implements it. This lead to a mapping between a CPU model and
whether it supports split lock detection by default (no need to check
IA32_CORE_CAPABILITIES) or they may support the split lock detection
and IA32_CORE_CAPABILITIES bit 5 has to be checked.

With more and more CPU models, this becomes hard to maintain.
Moreover, the December 2022 edition of the Intel Instruction Set
Extensions manual defined that the split lock disable bit in the
IA32_CORE_CAPABILITIES MSR is (and retrospectively always has been)
architectural. Documentation also mentions “All processors that
enumerate support for MSR_IA32_CORE_CAPS and set
MSR_IA32_CORE_CAPS_SPLIT_LOCK_DETECT support split lock detection.”

Thus, commit d7ce15e1d4162ab5e56dead10d4ae69a6b5c8ee8:
“ x86/split_lock: Enumerate architectural split lock disable bit”
from linux-next removes the need of adding a new CPU model in that
mapping.

This is needed to support split lock detection for the new EMR CPU and
future models that don’t suffer architectural changes.

[Testing]
 Kernel was built on cbd and boot tested on a VM.
Intel was asked to install and test the new kernel from this ppa
https://launchpad.net/~roxanan/+archive/ubuntu/lp2015855

[Regression potential]
 Very low, it is a small refactor and in essence, it does the exact
 same thing for existing cpu models:
1. For Icelake which does not have IA32_CORE_CAPABILITIES it always
assumes the mechanism is supported.
2. For the rest, it automatically checks bit 5 of
IA32_CORE_CAPABILITIES without the extra step of checking the map if
the CPU supports this (which was always true)

Fenghua Yu (1):
  x86/split_lock: Enumerate architectural split lock disable bit

 arch/x86/kernel/cpu/intel.c | 58 ++++++++++++++-----------------------
 1 file changed, 22 insertions(+), 36 deletions(-)

-- 
2.34.1