Creating a 64-bit kernel: Difference between revisions
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Kernel virtual memory mappings in long mode: notes from a discussion with geist at #osdev |
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Long mode provides essentially an infinite amount of address space. An interesting design decision is how to map and use the kernel address space. Linux approaches the problem by permanently mapping the -2GB virtual region 0xffffffff80000000 -> 0xffffffffffffffff to physical address 0x0 upwards. Kernel data structures, which are usually allocated by kmalloc() and the slab allocator, reside above the 0xffffffff80000000 virtual base and are allocate from the physical 0 -> 2GB zone. This necessitates the ability of 'zoning' the page allocator, asking the page allocator to returning a page frame from a specific region, and only from that region. If a physical address above 2GB needs to accessed, the kernel temporarily map it to its space in a temporary mapping space below the virtual addresses base. The Linux approach provides the advantage of not having to modify the page tables much which means less TLB shootdowns on an SMP system.
Another approach is to treat the kernel address space as any other address space and dynamically map its regions. This provides the advantage of simplifying the page allocator by avoiding the need of physical memory 'zones': all physical RAM is available for any part of the kernel. An example of this approach is mapping the kernel to 0xfffffff800000000 as usual. Below that virtual address you put a large mapping for the entire physical address space, and use the virtual 0xfffffff800000000 -> 0xffffffffffffffff region above kernel memory area as a temporary mappings space.
== See Also ==
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