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    SubjectRe: Calling current() from interrupt context
    On Tue, Oct 10, 2000 at 12:36:35AM +0200, Andi Kleen wrote:
    > On Mon, Oct 09, 2000 at 11:30:50PM +0100, Alan Cox wrote:
    > > > I think I'll go for the 'current is in a well-known register'
    > > > approach and see how this goes...
    > >
    > > Failing that the 2.0 approach will work, current is a global in uniprocessor
    > > and a #define to an array indexed by cpu id in smp
    > The problem is where to get the cpuid from (see how smp_processor_id
    > is currently defined ;) When you don't have a hidden register in the
    > CPU you're screwed.
    > [x86-64 has one btw]

    Simple. Each interrupt stack is, say, 8 pages. You have an array
    of N interrupt stacks. Then you calculate

    cpu_id = (sp & ~(INT_STACK_SIZE-1)) >> (PAGE_SHIFT + 3);

    Actually, I'd put the interrupt stack and any other per-cpu data
    structures together in this region.

    I don't know yet how you decide which secondary processor is which
    at boot time. Maybe it doesn't matter, so you can just let them
    fight over the per-cpu data structures by trying to claim spinlocks
    on each one in turn.

    Anyway, this SMP stuff will be quite academic for a while unless
    someone wants to donate a workstation-sized SMP VAX (if such a
    beast exists at all :-)


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