linux-kernel - Re: [PATCH 10/27] KVM: MMU: Add infrastructure for two-level page walker

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Date:	Wed, 8 Sep 2010 11:20:14 +0200
From:	"Roedel, Joerg" <Joerg.Roedel@....com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	Alexander Graf <agraf@...e.de>,
	"joro@...tes.org" <joro@...tes.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/27] KVM: MMU: Add infrastructure for two-level page
 walker

On Mon, Sep 06, 2010 at 02:05:35PM -0400, Avi Kivity wrote:
>   On 09/06/2010 06:55 PM, Joerg Roedel wrote:
> > This patch introduces a mmu-callback to translate gpa
> > addresses in the walk_addr code. This is later used to
> > translate l2_gpa addresses into l1_gpa addresses.
> 
> > @@ -534,6 +534,11 @@ static inline gpa_t gfn_to_gpa(gfn_t gfn)
> >   	return (gpa_t)gfn<<  PAGE_SHIFT;
> >   }
> >
> > +static inline gfn_t gpa_to_gfn(gpa_t gpa)
> > +{
> > +	return (gfn_t)gpa>>  PAGE_SHIFT;
> > +}
> > +
> 
> That's a bug - gfn_t may be smaller than gpa_t, so you're truncating 
> just before the shift.  Note the casts in the surrounding functions are 
> widening, not narrowing.
> 
> However, gfn_t is u64 so the bug is only theoretical.

Will fix that in v4 too. Thanks.

	Joerg

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