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

Zachary Amsden
2007-03-21T01:53:31Z

Linus Torvalds wrote:
> 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.
>

Actually, I was thinking the irq handlers would just not mess around
with eflags on the stack, just call the chip to ack the interrupt and
re-enable hardware interrupts when they left, since that is free anyway
with the iret. Maybe leaving irqs disabled is better.

> 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?
>

I wasn't disabling preemption to actually disable preemption. I was
just using bh_disable as a global hammer to stop softirqs (thus the irq
replay tasklet) from running during the normal irq_exit path. Then, we
can just use the existing software IRQ replay code, and I think barely
any new code (queue_irq(), etc) has to be written.

Zach