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    SubjectRe: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops callsites to make them patchable
    On Tue, Mar 20, 2007 at 09:31:58AM -0700, Jeremy Fitzhardinge wrote:
    > Linus Torvalds wrote:
    > > On Tue, 20 Mar 2007, Eric W. Biederman wrote:
    > >
    > >> If that is the case. In the normal kernel what would
    > >> the "the oops, we got an interrupt code do?"
    > >> I assume it would leave interrupts disabled when it returns?
    > >> Like we currently do with the delayed disable of normal interrupts?
    > >>
    > >
    > > Yeah, disable interrupts, and set a flag that the fake "sti" can test, and
    > > just return without doing anything.
    > >
    > > (You may or may not also need to do extra work to Ack the hardware
    > > interrupt etc, which may be irq-controller specific. Once the CPU has
    > > accepted the interrupt, you may not be able to just leave it dangling)
    > >
    >
    > So it would be something like:
    >
    > pda.intr_mask = 1; /* disable interrupts */
    > ...
    > pda.intr_mask = 0; /* enable interrupts */
    > if (xchg(&pda.intr_pending, 0)) /* check pending */
    > asm("sti"); /* was pending; isr left cpu interrupts masked */

    I don't know that you need an xchg there. If you're still on the same
    CPU, it should all be nice and causal even across an interrupt handler.
    So it could be:

    pda.intr_mask = 0; /* intr_pending can't get set after this */
    if (unlikely(pda.intr_pending)) {
    pda.intr_pending = 0;
    asm("sti");
    }

    (This would actually need a C barrier, but I'll ignore that as this'd
    end up being asm...)

    But other interesting things could happen. If we never did a real CLI
    and we get preempted and switched to another CPU between clearing
    intr_mask and checking intr_pending, we get a little confused.

    But perhaps that doesn't matter because we'd by definition have no
    pending interrupts on either processor?

    Is it expensive to do an STI if interrupts are already enabled?

    --
    Mathematics is the supreme nostalgia of our time.
    -
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