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

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Message-ID: <20070320224324.GL10459@waste.org>
Date:	Tue, 20 Mar 2007 17:43:24 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Zachary Amsden <zach@...are.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, Mar 20, 2007 at 09:31:58AM -0700, Jeremy Fitzhardinge wrote:
> Linus Torvalds wrote:
> > On Tue, 20 Mar 2007, Eric W. Biederman wrote:
> >   
> >> If that is the case.  In the normal kernel what would
> >> the "the oops, we got an interrupt code do?"
> >> I assume it would leave interrupts disabled when it returns?
> >> Like we currently do with the delayed disable of normal interrupts?
> >>     
> >
> > Yeah, disable interrupts, and set a flag that the fake "sti" can test, and 
> > just return without doing anything.
> >
> > (You may or may not also need to do extra work to Ack the hardware 
> > interrupt etc, which may be irq-controller specific. Once the CPU has 
> > accepted the interrupt, you may not be able to just leave it dangling)
> >   
> 
> So it would be something like:
> 
>     pda.intr_mask = 1;		/* disable interrupts */
>     ...
>     pda.intr_mask = 0;		/* enable interrupts */
>     if (xchg(&pda.intr_pending, 0))	/* check pending */
>     	asm("sti");		/* was pending; isr left cpu interrupts masked */

I don't know that you need an xchg there. If you're still on the same
CPU, it should all be nice and causal even across an interrupt handler.
So it could be:

   pda.intr_mask = 0; /* intr_pending can't get set after this */
   if (unlikely(pda.intr_pending)) {
      pda.intr_pending = 0;
      asm("sti");
   }

(This would actually need a C barrier, but I'll ignore that as this'd
end up being asm...)

But other interesting things could happen. If we never did a real CLI
and we get preempted and switched to another CPU between clearing
intr_mask and checking intr_pending, we get a little confused. 

But perhaps that doesn't matter because we'd by definition have no
pending interrupts on either processor?

Is it expensive to do an STI if interrupts are already enabled?

-- 
Mathematics is the supreme nostalgia of our time.
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