Hi Everyone,

We merged the new ID sysreg management scheme into RMM today.

Background:

Previously, RMM directly read the ID_xxx sysregs to determine hardware capabilities before enabling features in the Realm world. This approach implicitly required EL3 to enable the feature first, creating a revision lockstep between RMM and the EL3 firmware.

New Scheme:

With the updated ID register management, EL3 capabilities are queried via the SMCCC_ARCH_FEATURE_AVAILABILITY call. RMM now maintains a shadow copy of the ID sysregs in memory (referred to in the codebase as the cached_ID reg). This cached copy is populated during cold boot, and RMM will only access the cached ID regs at runtime.

The design also anticipates future support for FEAT_IDTE3 in TF-A.

Benefits:

• Sanitized Realm view of ID regs: Instead of using a “disallow” mask, RMM now uses an “allow” mask to define what features are exposed.,
• Forward compatibility: Older RMM versions running on newer architectures will not expose previously RES0 fields to realms, thanks to the “allow” mask approach.,
• Feature discrepancy handling: In the future, RMM could detect CPU feature differences and react accordingly.,
• Live migration readiness: This scheme may also help when handling feature differences in live migration scenarios.,

The merge commit can be found here : https://github.com/TF-RMM/tf-rmm/commit/4a9d781892074e8997e73411cfe5a220223417dc

The current CI tests are passing. There is a chance that we may not have exposed all the bitfields needed, but if you find any failure due to missing ID features, please let us know.

Please be aware that outstanding patches may need to be rebased on top of this.

Best Regards

Soby Mathew