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I have been reviewing how OP‑TEE is implemented and how secure‑world invocations behave. The goal was to determine whether an OP‑TEE call can delay the Linux side and introduce latency depending on the time spent in the secure world. Similar latency effects are already known for EFI runtime services, but this was not documented. To mitigate the impact, EFI runtime invocations can be restricted to specific CPUs so that real‑time workloads on other CPUs remain unaffected. This mechanism, however, is only described in the commit that introduced it. This change adds a firmware section that documents these behaviours explicitly. It highlights cases where firmware can delay the kernel, information that may be unfamiliar to some users and surprising-or concerning-to others. Assisted-by: Microsoft-Copilot Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: Jonathan Corbet <corbet@lwn.net> Message-ID: <20260718175041.QXn9iOFK@linutronix.de>