Virtual Memory

Virtual memory (VM) in Zephyr provides developers with the ability to fine tune access to memory. To utilize virtual memory, the platform must support Memory Management Unit (MMU) and it must be enabled in the build. Due to the target of Zephyr mainly being embedded systems, virtual memory support in Zephyr differs a bit from that in traditional operating systems:

Mapping of Kernel Image

Default is to do 1:1 mapping for the kernel image (including code and data) between physical and virtual memory address spaces, if demand paging is not enabled. Deviation from this requires careful manipulation of linker script.

Secondary Storage

Basic virtual memory support does not utilize secondary storage to extend usable memory. The maximum usable memory is the same as the physical memory.

  • Demand Paging enables utilizing secondary storage as a backing store for virtual memory, thus allowing larger usable memory than the available physical memory. Note that demand paging needs to be explicitly enabled.

  • Although the virtual memory space can be larger than physical memory space, without enabling demand paging, all virtually mapped memory must be backed by physical memory.

Kconfigs

Required

These are the Kconfigs that need to be enabled or defined for kernel to support virtual memory.

Optional

  • CONFIG_KERNEL_DIRECT_MAP: permits 1:1 mappings between virtual and physical addresses, instead of kernel choosing addresses within the virtual address space. This is useful for mapping device MMIO regions for more precise access control.

Memory Map Overview

This is an overview of the memory map of the virtual memory address space. Note that the Z_* macros, which are used in code, may have different meanings depending on architecture and Kconfigs, which will be explained below.

+--------------+ <- Z_VIRT_RAM_START
| Undefined VM | <- architecture specific reserved area
+--------------+ <- Z_KERNEL_VIRT_START
| Mapping for  |
| main kernel  |
| image        |
|              |
|              |
+--------------+ <- Z_FREE_VM_START
|              |
| Unused,      |
| Available VM |
|              |
|..............| <- grows downward as more mappings are made
| Mapping      |
+--------------+
| Mapping      |
+--------------+
| ...          |
+--------------+
| Mapping      |
+--------------+ <- memory mappings start here
| Reserved     | <- special purpose virtual page(s) of size Z_VM_RESERVED
+--------------+ <- Z_VIRT_RAM_END

  • Z_VIRT_RAM_START is the beginning of the virtual memory address space. This needs to be page aligned. Currently, it is the same as CONFIG_KERNEL_VM_BASE.

  • Z_VIRT_RAM_SIZE is the size of the virtual memory address space. This needs to be page aligned. Currently, it is the same as CONFIG_KERNEL_VM_SIZE.

  • Z_VIRT_RAM_END is simply (Z_VIRT_RAM_START + Z_VIRT_RAM_SIZE).

  • Z_KERNEL_VIRT_START is the same as z_mapped_start specified in the linker script. This is the virtual address of the beginning of the kernel image at boot time.

  • Z_KERNEL_VIRT_END is the same as z_mapped_end specified in the linker script. This is the virtual address of the end of the kernel image at boot time.

  • Z_FREE_VM_START is the beginning of the virtual address space where addresses can be allocated for memory mapping. This depends on whether CONFIG_ARCH_MAPS_ALL_RAM is enabled.

    • If it is enabled, which means all physical memory are mapped in virtual memory address space, and it is the same as (CONFIG_SRAM_BASE_ADDRESS + CONFIG_SRAM_SIZE).

    • If it is disabled, Z_FREE_VM_START is the same Z_KERNEL_VIRT_END which is the end of the kernel image.

  • Z_VM_RESERVED is an area reserved to support kernel functions. For example, some addresses are reserved to support demand paging.

Virtual Memory Mappings

Setting up Mappings at Boot

In general, most supported architectures set up the memory mappings at boot as following:

  • .text section is read-only and executable. It is accessible in both kernel and user modes.

  • .rodata section is read-only and non-executable. It is accessible in both kernel and user modes.

  • Other kernel sections, such as .data, .bss and .noinit, are read-write and non-executable. They are only accessible in kernel mode.

    • Stacks for user mode threads are automatically granted read-write access to their corresponding user mode threads during thread creation.

    • Global variables, by default, are not accessible to user mode threads. Refer to Memory Domains and Partitions on how to use global variables in user mode threads, and on how to share data between user mode threads.

Caching modes for these mappings are architecture specific. They can be none, write-back, or write-through.

Note that SoCs have their own additional mappings required to boot where these mappings are defined under their own SoC configurations. These mappings usually include device MMIO regions needed to setup the hardware.

Mapping Anonymous Memory

The unused physical memory can be mapped in virtual address space on demand. This is conceptually similar to memory allocation from heap, but these mappings must be aligned on page size and have finer access control.

  • k_mem_map() can be used to map unused physical memory:

    • The requested size must be multiple of page size.

    • The address returned is inside the virtual address space between Z_FREE_VM_START and Z_VIRT_RAM_END.

    • The mapped region is not guaranteed to be physically contiguous in memory.

    • Guard pages immediately before and after the mapped virtual region are automatically allocated to catch access issue due to buffer underrun or overrun.

  • The mapped region can be unmapped (i.e. freed) via k_mem_unmap():

    • Caution must be exercised to give the pass the same region size to both k_mem_map() and k_mem_unmap(). The unmapping function does not check if it is a valid mapped region before unmapping.

API Reference

Kernel Memory Management