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

Zachary Amsden
2007-03-20T23:08:21Z

Matt Mackall wrote:
> 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 */
>

Why not? Oh, I see. intr_mask is inverted form of EFLAGS_IF.

> 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.
>

I think Jeremy's idea was to have interrupt handlers leave interrupts
disabled on exit if pda.intr_mask was set. In which case, they would
bypass all work and we could never get preempted. I don't think leaving
hardware interrupts disabled for such a long time is good though.

> 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?
>

Yes.

Zach