Hi Everyone,

We are planning a major change to granule scub flow in TF-RMM. This is done mainly in preparation for supporting FEAT_MEC. When FEAT_MEC is present, granules must be zeroed with the appropriate MECID programmed. Previously, RMM performed scrubbing (zeroing) during the transition to the DELEGATED state. However, for FEAT_MEC, granules must be initialized with the correct MECID when they transition into a Realm. The new granule transition flow improves efficiency and eliminates redundant operations while preserving security guarantees.

Key changes:

• Scrubbing now occurs during transitions from DELEGATED to another state, instead of to DELEGATED.,
• This ensures that data is sanitized before a granule is either:   Assigned to a new Realm, or reclaimed by the NS Host.,

All RMI_*_CREATE() APIs, which transition granules from DELEGATED to Realm states, and RMI_UNDELEGATE() have been updated to correctly initialize or zero the buffer as needed.

Performance improvements:

• Redundant zeroing previously done during RMI_DATA_CREATE() and RMI_RTT_CREATE() is eliminated.,
• When FEAT_MEC is enabled, scrubbing before initialization (for new Realm usage) is removed, further improving efficiency.,

The patch is here : https://review.trustedfirmware.org/c/TF-RMM/tf-rmm/+/39096/22 . Please let us know if you see some issues with the change.
The FEAT_MEC patch series is available for review here : https://review.trustedfirmware.org/c/TF-RMM/tf-rmm/+/35509/9

Best Regards

Soby Mathew