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

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)

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.


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