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Open real-time systems provide for co-hosting hard-, soft- and non-real-time applications. Microkernel-based designs in addition allow for these applications to be mutually protected. Thus, trusted servers can coexist next to untrusted applications. These systems place a heavy burden on the performance of the message-passing mechanism, especially when based on microkernel-like inter-process communication. In this paper we introduce capacity-reserve donation (in short Credo), a mechanism for the fast interaction of interdependent components, which is applicable to common real-time resource-access models. We implemented Credo by extending L4's message-passing mechanism to provide proper resource accounting and time-donation control, thereby preserving desired real-time properties. We were able to achieve priority inheritance and stack-based priority-ceiling resource sharing with virtually no …