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MPU Stack Objects

Thread Stack Creation

Thread stacks are declared statically with K_THREAD_STACK_DEFINE() or embedded within structures using K_THREAD_STACK_MEMBER()

For architectures which utilize memory protection unit (MPU) hardware, stacks are physically contiguous allocations. This contiguous allocation has implications for the placement of stacks in memory, as well as the implementation of other features such as stack protection and userspace. The implications for placement are directly attributed to the alignment requirements for MPU regions. This is discussed in the memory placement section below.

Stack Guards

Stack protection mechanisms require hardware support that can restrict access to memory. Memory protection units can provide this kind of support. The MPU provides a fixed number of regions. Each region contains information about the start, end, size, and access attributes to be enforced on that particular region.

Stack guards are implemented by using a single MPU region and setting the attributes for that region to not allow write access. If invalid accesses occur, a fault ensues. The stack guard is defined at the bottom (the lowest address) of the stack.

Memory Placement

During stack creation, a set of constraints are enforced on the allocation of memory. These constraints include determining the alignment of the stack and the correct sizing of the stack. During linking of the binary, these constraints are used to place the stacks properly.

The main source of the memory constraints is the MPU design for the SoC. The MPU design may require specific constraints on the region definition. These can include alignment of beginning and end addresses, sizes of allocations, or even interactions between overlapping regions.

Some MPUs require that each region be aligned to a power of two. These SoCs will have CONFIG_MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT defined. This means that a 1500 byte stack should be aligned to a 2kB boundary and the stack size should also be adjusted to 2kB to ensure that nothing else is placed in the remainder of the region. SoCs which include the unmodified ARM v7m MPU will have these constraints.

Some ARM MPUs use start and end addresses to define MPU regions and both the start and end addresses require 32 byte alignment. An example of this kind of MPU is found in the NXP FRDM K64F.

MPUs may have a region priority mechanisms that use the highest priority region that covers the memory access to determine the enforcement policy. Others may logically OR regions to determine enforcement policy.

Size and alignment constraints may result in stack allocations being larger than the requested size. Region priority mechanisms may result in some added complexity when implementing stack guards.