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

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Date:	Tue, 20 Mar 2007 17:24:58 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Zachary Amsden <zach@...are.com>
cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Rusty Russell <rusty@...tcorp.com.au>, Andi Kleen <ak@....de>,
	David Miller <davem@...emloft.net>, mingo@...e.hu,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.osdl.org, xen-devel@...ts.xensource.com,
	chrisw@...s-sol.org, anthony@...emonkey.ws, netdev@...r.kernel.org
Subject: Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops
 callsites to make them patchable



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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