From mboxrd@z Thu Jan  1 00:00:00 1970
From: Scott Wood <scottwood@freescale.com>
Subject: Re: [PATCH 2/4] KVM: PPC: Book3S HV: Add a mechanism for recording
 modified HPTEs
Date: Mon, 19 Nov 2012 11:20:38 -0600
Message-ID: <1353345638.10768.1@snotra>
References: <20121114043034.GA13832@drongo> <20121114043217.GC13832@drongo>
	<7B2C8404-8556-4A64-A20A-BDB85F22B94D@suse.de>
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Cc: Paul Mackerras <paulus@samba.org>, <kvm@vger.kernel.org>,
	<kvm-ppc@vger.kernel.org>
To: Alexander Graf <agraf@suse.de>
Return-path: <kvm-ppc-owner@vger.kernel.org>
In-Reply-To: <7B2C8404-8556-4A64-A20A-BDB85F22B94D@suse.de> (from
	agraf@suse.de on Mon Nov 19 06:36:42 2012)
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Sender: kvm-ppc-owner@vger.kernel.org
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On 11/19/2012 06:36:42 AM, Alexander Graf wrote:
> 
> On 14.11.2012, at 05:32, Paul Mackerras wrote:
> 
> > +/*
> > + * Note modification of an HPTE; set the HPTE modified bit
> > + * if it wasn't modified before and anyone is interested.
> > + */
> > +static inline void note_hpte_modification(struct kvm *kvm,
> > +					  struct revmap_entry *rev)
> > +{
> > +	if (!(rev->guest_rpte & HPTE_GR_MODIFIED) &&
> > +	    atomic_read(&kvm->arch.hpte_mod_interest))
> 
> Aren't atomic reads expensive?

No, it's a simple load instruction (done with inline asm so the  
compiler can't break it up into smaller loads).

-Scott