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

[Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>]

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

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