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    SubjectRe: [rfc 08/45] cpu alloc: x86 support
    On Tue, 20 Nov 2007, Andi Kleen wrote:

    > > So I think we have a 2GB area right?
    >
    > For everything that needs the -31bit offsets; that is everything linked

    Of course.

    > > 1GB kernel
    > > 1GB - 1x per cpu area (128M?) modules?
    > > cpu aree 0
    > > ---- 2GB limit
    > > cpu area 1
    > > cpu area 2
    > > ....
    > >
    > > For that we would need to move the kernel down a bit. Can we do that?
    >
    > The kernel model requires kernel and modules and everything else
    > linked be in negative -31bit space. That is how the kernel code model is
    > defined.

    Right so I could move the kernel to

    #define __PAGE_OFFSET _AC(0xffff810000000000, UL)
    #define __START_KERNEL_map_AC(0xfffffff800000000, UL)
    #define KERNEL_TEXT_START _AC(0xfffffff800000000, UL) 30 bits = 1GB for kernel text
    #define MODULES_VADDR _AC(0xfffffff880000000, UL) 30 bits = 1GB for modules
    #define MODULES_END _AC(0xfffffff8f0000000, UL)
    #define CPU_AREA_BASE _AC(0xfffffff8f0000000, UL) 31 bits 256MB for cpu area 0
    #define CPU_AREA_BASE1 _AC(0xfffffff900000000, UL) More cpu areas for higher numbered processors
    #define CPU_AREA_END _AC(0xffffffffffff0000, UL)

    > You could in theory move the modules, but then you would need to implement
    > a full PIC dynamic linker for them first and also increase runtime overhead
    > for them because they would need to use a GOT/PLT.

    Why is it not possible to move the kernel lower while keeping bit 31 1?

    > I suspect all of this would cause far more overhead all over the kernel than
    > you could ever save with the per cpu data in your fast paths.

    Moving the kernel down a bit seems to be trivial without any of the weird
    solutions.

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