Kernel Memory Layout on ARM Linux

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                Kernel Memory Layout on ARM Linux
                Russell King
                     November 17, 2005 (2.6.15)
This document describes the virtual memory layout which the Linux
kernel uses for ARM processors.  It indicates which regions are
free for platforms to use, and which are used by generic code.
The ARM CPU is capable of addressing a maximum of 4GB virtual memory
space, and this must be shared between user space processes, the
kernel, and hardware devices.
As the ARM architecture matures, it becomes necessary to reserve
certain regions of VM space for use for new facilities; therefore
this document may reserve more VM space over time.
Start                End                Use
--------------------------------------------------------------------------
ffff8000        ffffffff        copy_user_page / clear_user_page use.
                                For SA11xx and Xscale, this is used to
                                setup a minicache mapping.
ffff1000        ffff7fff        Reserved.
                                Platforms must not use this address range.
ffff0000        ffff0fff        CPU vector page.
                                The CPU vectors are mapped here if the
                                CPU supports vector relocation (control
                                register V bit.)
ffc00000        fffeffff        DMA memory mapping region.  Memory returned
                                by the dma_alloc_xxx functions will be
                                dynamically mapped here.
ff000000        ffbfffff        Reserved for future expansion of DMA
                                mapping region.
VMALLOC_END        feffffff        Free for platform use, recommended.
                                VMALLOC_END must be aligned to a 2MB
                                boundary.
VMALLOC_START        VMALLOC_END-1        vmalloc() / ioremap() space.
                                Memory returned by vmalloc/ioremap will
                                be dynamically placed in this region.
                                VMALLOC_START may be based upon the value
                                of the high_memory variable.
PAGE_OFFSET        high_memory-1        Kernel direct-mapped RAM region.
                                This maps the platforms RAM, and typically
                                maps all platform RAM in a 1:1 relationship.
TASK_SIZE        PAGE_OFFSET-1        Kernel module space
                                Kernel modules inserted via insmod are
                                placed here using dynamic mappings.
00001000        TASK_SIZE-1        User space mappings
                                Per-thread mappings are placed here via
                                the mmap() system call.
00000000        00000fff        CPU vector page / null pointer trap
                                CPUs which do not support vector remapping
                                place their vector page here.  NULL pointer
                                dereferences by both the kernel and user
                                space are also caught via this mapping.
Please note that mappings which collide with the above areas may result
in a non-bootable kernel, or may cause the kernel to (eventually) panic
at run time.
Since future CPUs may impact the kernel mapping layout, user programs
must not access any memory which is not mapped inside their 0x0001000
to TASK_SIZE address range.  If they wish to access these areas, they
must set up their own mappings using open() and mmap().


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