Specifications

This is the Title of the Book, eMatter Edition

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Chapter 15: Memory Mapping and DMA

physical addresses differ only by a constant offset. Logical addresses use the

hardware’s native pointer size and, therefore, may be unable to address all of

physical memory on heavily equipped 32-bit systems. Logical addresses are usu-

ally stored in variables of type

unsigned long or void *. Memory returned from

kmalloc has a kernel logical address.

Kernel virtual addresses

Kernel virtual addresses are similar to logical addresses in that they are a map-

ping from a kernel-space address to a physical address. Kernel virtual addresses

do not necessarily have the linear, one-to-one mapping to physical addresses that

characterize the logical address space, however. All logical addresses are kernel

virtual addresses, but many kernel virtual addresses are not logical addresses.

For example, memory allocated by vmalloc has a virtual address (but no direct

physical mapping). The kmap function (described later in this chapter) also

returns virtual addresses. Virtual addresses are usually stored in pointer variables.

If you have a logical address, the macro __pa( ) (defined in <asm/page.h>) returns its

associated physical address. Physical addresses can be mapped back to logical

addresses with __va( ), but only for low-memory pages.

Figure 15-1. Address types used in Linux

kernel virtual

addresses

kernel logical

addresses

high memory

low memory

user process

Key

physical memory address space

page mapping

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