linux-kernel - Re: [PATCH 0/2] jump label: 2.6.38 updates

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Message-ID: <20110216083542.GE16529@elte.hu>
Date:	Wed, 16 Feb 2011 09:35:42 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	David Miller <davem@...emloft.net>, simoneau@....uri.edu,
	will.newton@...il.com, hpa@...or.com, matt@...sole-pimps.org,
	rostedt@...dmis.org, peterz@...radead.org, jbaron@...hat.com,
	mathieu.desnoyers@...ymtl.ca, tglx@...utronix.de,
	andi@...stfloor.org, roland@...hat.com, rth@...hat.com,
	masami.hiramatsu.pt@...achi.com, fweisbec@...il.com,
	avi@...hat.com, sam@...nborg.org, ddaney@...iumnetworks.com,
	michael@...erman.id.au, linux-kernel@...r.kernel.org,
	vapier@...too.org, cmetcalf@...era.com, dhowells@...hat.com,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com
Subject: Re: [PATCH 0/2] jump label: 2.6.38 updates


* Benjamin Herrenschmidt <benh@...nel.crashing.org> wrote:

> On Tue, 2011-02-15 at 13:27 -0800, David Miller wrote:
> > From: Will Simoneau <simoneau@....uri.edu>
> > Date: Tue, 15 Feb 2011 16:11:23 -0500
> > 
> > > Note how the cache and cache coherence protocol are fundamental parts of this
> > > operation; if these instructions simply bypassed the cache, they *could not*
> > > work correctly - how do you detect when the underlying memory has been
> > > modified?
> > 
> > The issue here isn't L2 cache bypassing, it's local L1 cache bypassing.
> > 
> > The chips in question aparently do not consult the local L1 cache on
> > "ll" instructions.
> > 
> > Therefore you must only ever access such atomic data using "ll"
> > instructions.
> 
> Note that it's actually a reasonable design choice to not consult the L1
> in these case .... as long as you invalidate it on the way. That's how
> current powerpcs do it afaik, they send a kill to any matching L1 line
> along as reading from the L2. (Of course, L1 has to be write-through for
> that to work).

Just curious: how does this work if there's an interrupt (or NMI) right after the 
invalidate instruction but before the 'll' instruction? The IRQ/NMI may refill the 
L1. Or are the two instructions coupled by hw (they form a single instruction in 
essence) and irqs/NMIs are inhibited inbetween?

Thanks,

	Ingo
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