2.5.4. Security and the PMU

You can use both Secure and Non-secure transactions to access the PMU registers. However, you can configure the PMU to count only Non-secure events, or both Secure and Non-secure events, depending on the SPNIDEN input. The default is Non-secure events.

If the SPNIDEN input is taken HIGH, there is a potential security risk because Non-secure software can observe Secure activity through the performance counters. See Appendix A Signal Descriptions. ARM advises that you consider the security to be breached for devices placed in this state and that you take appropriate action.

If the SPNIDEN input goes from HIGH to LOW, that is, the PMUs go from counting all events to counting only Non-secure events, the counters can contain information relating to Secure transactions. Therefore, ARM recommends that software sets the counters to zero if access to that information might represent a potential security risk.

Note

Unlike ARM processors, SPNIDEN applies to events from both user and privileged transactions and the CCI-400 makes no distinction between them.

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