The IPI registers can only be configured by a TrustZone secure transaction. The secure transaction is routed through the LPD_XPPU protection unit to make sure the transaction host has access privileges before it is allowed to reach the IPI programming interface with its additional restrictions.
Writes to the IPI registers can be blocked by setting the LOCK [ReqWrDis] lock bit = 1. Once this bit is set, it can only be cleared by a POR.
After the lock bit is set, many of the registers can no longer be written to until a POR occurs. The lockability of the registers are shown in the Control Registers table.