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[Xen-devel] Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravi

To: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops callsites to make them patchable
From: Zachary Amsden <zach@xxxxxxxxxx>
Date: Tue, 20 Mar 2007 18:53:54 -0800
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, akpm@xxxxxxxxxxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxx, netdev@xxxxxxxxxxxxxxx, Rusty Russell <rusty@xxxxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, chrisw@xxxxxxxxxxxx, Andi Kleen <ak@xxxxxx>, "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>, anthony@xxxxxxxxxxxxx, mingo@xxxxxxx, David Miller <davem@xxxxxxxxxxxxx>
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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

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