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

Linus Torvalds
2007-03-21T00:26:26Z

On Tue, 20 Mar 2007, Zachary Amsden wrote:
>
> void local_irq_restore(int enabled)
> {
> pda.intr_mask = enabled;
> /*
> * note there is a window here where softirqs are not processed by
> * the interrupt handler, but that is not a problem, since it will
> * get done here in the outer enable of any nested pair.
> */
> if (enabled)
> local_bh_enable();
> }

Actually, this one is more complicated. You also need to actually enable
hardware interrupts again if they got disabled by an interrupt actually
occurring while the "soft-interrupt" was disabled.

But since it's all a local-cpu issue, you can do things like test
cpu-local memory flags for whetehr that has happened or not.

So it *should* be something as simple as

local_irq_disable()
{
pda.irq_enable = 0;
}

handle_interrupt()
{
if (!pda.irq_enable) {
pda.irq_queued = 1;
queue_interrupt();
.. make sure we return with hardirq's now
disabled: just clear IF in the pt_regs ..
return;
}
.. normal ..
}

local_irq_enable()
{
pda.irq_enable = 1;
barrier();
/* Common case - nothing happened while we were fake-disabled.. */
if (!pda.irq_queued)
return;

/* Ok, actually handle the things! */
handle_queued_irqs();

/*
* And enable the hw interrupts again, they got disabled
* when we were queueing stuff..
*/
hardware_sti();
}

but I haven't really gone over it in any detail, I may have missed
something really obvious.

Anyway, it really *should* be pretty damn simple. No need to disable
preemption, there should be no events that can *cause* it, since all
interrupts get headed off at the pass.. (the return-from-interrupt thng
should already notice that it's returning to an interrupts-disabled
section and not try to do any preemption).

What did I miss?

Linus