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    SubjectRe: [PATCH] x86: push old stack address on irqstack for unwinder

    On Sat, Jan 31, 2009 at 01:39:21AM +0100, Ingo Molnar wrote:
    >
    > * H. Peter Anvin <hpa@zytor.com> wrote:
    >
    > > H. Peter Anvin wrote:
    > >> Martin Hicks wrote:
    > >>> Hi,
    > >>>
    > >>> KDB was using this information. Could this be pushed towards 2.6.29
    > >>> please?
    > >>>
    > >>> This re-adds the old stack pointer to the top of the irqstack to help
    > >>> with unwinding. It was removed in commit
    > >>> d99015b1abbad743aa049b439c1e1dede6d0fa49
    > >>> as part of the save_args out-of-line work.
    > >>>
    > >>
    > >> This bothers me... why should we add even a single instruction to what
    > >> is arguably the single hottest path in the kernel to support an
    > >> out-of-tree debugger, especially if kgdb (which is in-tree) doesn't
    > >> need it?
    > >>
    > >> What does kgdb do differently (or is kgdb broken too)?
    > >>

    I was searching around, trying to find out if there was another way for
    kdb to do this, and I think removing the backlink is breaking other
    stuff also. dump_trace() in dumpstack_64.S is using the same trick as
    KDB to trace out of the interrupt stack:

    /*
    * We link to the next stack (which would be
    * the process stack normally) the last
    * pointer (index -1 to end) in the IRQ stack:
    */
    stack = (unsigned long *) (irqstack_end[-1]);
    irqstack_end = NULL;
    ops->stack(data, "EOI");
    continue;


    mh



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