How do you use the ARM MMU to isolate and protect different memory regions?

3 MMU attributes

The MMU attributes are the properties that define how a memory region is accessed and treated by the processor. The attributes include the memory type, the cacheability, the shareability, and the access permissions. The memory type determines how the memory behaves in terms of ordering, coherency, and atomicity. The cacheability determines whether the memory can be cached by the processor or not. The shareability determines whether the memory can be shared by multiple processors or not. The access permissions determine whether the memory can be read, written, or executed by different types of software.

4 MMU protection

The MMU protection is the mechanism that enforces the access permissions and prevents unauthorized or malicious access to memory regions. The MMU protection is based on the concept of exception levels, which are the privilege levels that the processor operates in. The ARM architecture defines four exception levels, from EL0 (least privileged) to EL3 (most privileged). The MMU protection checks the current exception level and the access permissions of the memory region, and generates a fault if there is a violation. The fault can be handled by software or hardware, depending on the configuration.

5 MMU isolation

The MMU isolation is the technique that uses the MMU to separate different memory regions and prevent interference or leakage between them. The MMU isolation is useful for implementing security features, such as secure boot, trusted execution environment, or hypervisor. The MMU isolation can be achieved by using different translation tables for different exception levels, or by using address space identifiers (ASIDs) to distinguish between different contexts. The MMU isolation can also be enhanced by using hardware extensions, such as the ARM TrustZone or the ARM virtualization extensions.

6 MMU optimization

The MMU optimization is the process of tuning the MMU configuration and attributes to improve the performance and efficiency of the memory system. The MMU optimization can involve choosing the optimal size and level of translation tables, adjusting the cacheability and shareability attributes, or using features such as memory domains or memory attributes indirection register (MAIR) to fine-tune the memory behavior. The MMU optimization can also benefit from using hardware features, such as the translation lookaside buffer (TLB) or the prefetch engine, to speed up the translation and access operations.