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

[Matt Mackall <mpm@xxxxxxxxxxx>]

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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