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 → 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)
Waiting on intel response if these 2 commits
- b041b525dab95
- 727209376f499
are needed to support this as well.