Date: Wed, 16 Feb 2011 09:35:42 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: David Miller <davem@davemloft.net>, simoneau@ele.uri.edu,
        will.newton@gmail.com, hpa@zytor.com, matt@console-pimps.org,
        rostedt@goodmis.org, peterz@infradead.org, jbaron@redhat.com,
        mathieu.desnoyers@polymtl.ca, tglx@linutronix.de, andi@firstfloor.org,
        roland@redhat.com, rth@redhat.com, masami.hiramatsu.pt@hitachi.com,
        fweisbec@gmail.com, avi@redhat.com, sam@ravnborg.org,
        ddaney@caviumnetworks.com, michael@ellerman.id.au,
        linux-kernel@vger.kernel.org, vapier@gentoo.org, cmetcalf@tilera.com,
        dhowells@redhat.com, schwidefsky@de.ibm.com, heiko.carstens@de.ibm.com
Subject: Re: [PATCH 0/2] jump label: 2.6.38 updates
Message-ID: <20110216083542.GE16529@elte.hu>
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 <20110215211123.GA3094@ele.uri.edu>
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* Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:

> On Tue, 2011-02-15 at 13:27 -0800, David Miller wrote:
> > From: Will Simoneau <simoneau@ele.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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