S-mode 2
S-mode 2 (or S-Mode 2) refers to a specific operational mode within the RISC-V instruction set architecture (ISA). It is a privileged mode designed to provide a secure environment for running trusted software, typically a hypervisor or security monitor.
S-mode 2 builds upon the standard Supervisor mode (S-mode) by adding enhanced isolation and security features. These enhancements are aimed at preventing software running in S-mode 1 (the standard Supervisor mode) from directly accessing or interfering with the operation of the software running in S-mode 2. This isolation is critical for virtualization and other security-sensitive applications.
The primary features of S-mode 2 typically include:
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Memory Protection: S-mode 2 usually has its own distinct memory protection scheme, preventing S-mode 1 from accessing its memory regions. This is generally achieved through separate page tables or other memory management unit (MMU) configurations.
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Interrupt and Exception Handling: Interrupts and exceptions can be configured to be handled differently in S-mode 2 than in S-mode 1. S-mode 2 can have its own interrupt controller or vector table, ensuring that critical interrupts are routed directly to the secure environment.
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Privileged Instructions: S-mode 2 often has access to a set of privileged instructions that are not available to S-mode 1. These instructions are used for tasks such as managing the MMU, configuring the interrupt controller, and performing other security-critical operations.
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Hardware Resource Partitioning: S-mode 2 may facilitate the partitioning of hardware resources, such as caches and peripherals, to prevent interference from S-mode 1. This helps to improve the security and reliability of the secure environment.
The existence and specific implementation of S-mode 2 are dependent on the specific RISC-V implementation and are not a mandatory part of the RISC-V ISA. Its use is primarily driven by the need for more robust security and isolation in applications such as virtualization and trusted execution environments (TEEs). The details of how S-mode 2 is implemented, including the specific hardware and software mechanisms used, will vary depending on the target platform and the specific security requirements.