Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops callsites to make them patchable

Matt Mackall
2007-03-20T22:57:45Z

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.