Address Types in Linux Kernel

Different Linux kernel functions require different type of addresses.

The following is a list of address types used in Linux

User virtual addresses: seen in user programs. User addresses are either 32 or 64 bits length, each process has its own virtual address space.

Physical addresses: (a better definition of physical addresses here)

Bus addresses: These are used between the peripheral buses and memory. They are the same as the physical addresses used by processor, but that’s not necessarily the case. Some architectures can provide an I/O memory management unit (IOMMU) that remaps addresses between a bus and main memory.

Kernel logical addresses: these make up the normal address space of the kernel (a definition of logical address here). All Linux process running in user land and kernel mode use the same segments to address instructions and data. Such segments start at 0x00000000 and reach 2 exp (32 -1) , so logical addresses coincide with the corresponding linear addresses.

Kernel virtual addresses: Also known as linear addresses. All logical addresses are kernel virtual addresses, but many kernel virtual addresses are not logical addresses.

Additional details

Linux kernel on x86 splits the 4GB as follows (This configuration can be change at kernel configuration time).Addres_space_linux
The maximum amount of physical memory that could be handled by the kernel is the amount that could be mapped into kernel’s portion of the virtual address space, minus the space needed for the kernel code itself.

Important notes:

Low memory: Logical addresses that exists in kernel space.  Kernel cannot directly manipulate memory that is not mapped into kernel’s address space.
High memory: Memory for which logical addresses do not exist, because of it is beyond the address range set aside for kernel virtual addresses. it tends to be reserved for user land processes. Before accessing a specific high memory page, the kernel must setup an explicit virtual mapping to make that page available in the kernel’s address space.





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