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    SubjectRe: kvm vmload/vmsave vs

    * Avi Kivity <> wrote:

    > Ingo Molnar wrote:
    >>> I think it's enough to switch %rsp before incrementing irqcount, no?
    >> no - that would introduce a small race: if an exception (say an NMI or
    >> MCE, or a debug trap) happens in that small window then the exception
    >> context thinks that it's on the IRQ stack already, and would use the
    >> task stack.
    > I'm suggesting
    > check irqcount
    > if (wasnt_in_irq)
    > rsp = irqstack
    > ++irqcount
    > If the NMI happens before the increment, we'll switch the stack
    > unconditionally, and if the NMI happens after the increment, then we
    > won't switch the stack, but we're guaranteed to be on the irqstack
    > anyway. The window size is negative :)
    > Similarly, the exit path should be
    > oldstack_reg = oldstack;
    > --irqcount;
    > rsp = oldstack_register;
    > To guarantee that by the time we decrement irqcount, we don't need the
    > stack anymore.

    agreed, something like this would work too. My suggestion, to eliminate
    irqcount altogether and just check RSP against the known-irqstack-range,
    could result in slightly smaller (and thus faster) code, but it's a
    marginal difference at best.


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      Last update: 2008-12-25 19:21    [W:0.022 / U:1.936 seconds]
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