linux-kernel - Re: [PATCH 05/12] xen: add m2p override mechanism

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Date:	Tue, 11 Jan 2011 12:44:39 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <Jeremy.Fitzhardinge@...rix.com>
Subject: Re: [PATCH 05/12] xen: add m2p override mechanism

On Mon, 10 Jan 2011, Konrad Rzeszutek Wilk wrote:
> > @@ -83,6 +85,14 @@ static inline unsigned long mfn_to_pfn(unsigned long mfn)
> >  	 */
> >  	__get_user(pfn, &machine_to_phys_mapping[mfn]);
> >  
> > +	/*
> > +	 * If this appears to be a foreign mfn (because the pfn
> > +	 * doesn't map back to the mfn), then check the local override
> > +	 * table to see if there's a better pfn to use.
> > +	 */
> > +	if (get_phys_to_machine(pfn) != mfn)
> 
> Any reason to not use 'pfn_to_mfn' call instead? If we do want
> to use the get_hhys_to_machine wouldn't be easier to just
> check for 'FOREIGN_FRAME_BIT' set? Or is just mostly a copy-n-paste
> of mfn_to_local_pfn?


pfn in this case is the return value of m2p(mfn); there are 3 possible
cases:

1) the mfn was a regular ram page belonging to the domain so
get_phys_to_machine(pfn) == mfn. Nothing to see here.

2) the mfn was a granted page so pfn is actually the pfn in the source
domain and doesn't mean anything in our domain. 
In this case get_phys_to_machine(pfn) is always different from mfn,
either because the mapping in our domain is just different or because
the FOREIGN_FRAME_BIT is set.

3) the pfn belongs to a 1:1 region. In this case
get_phys_to_machine(pfn) != mfn because of the IDENTITY_FRAME_BIT but we
don't actually want to look in the m2p_override because we are not going
to find anything useful there.


Considering all this, I propose this change:

if (get_phys_to_machine(pfn) != mfn)

into

if (pfn_to_mfn(pfn) != mfn)

because:

- it won't make any differences in case 1);

- it will avoid false potives in case 3);

- in case 2) if p2m(pfn) => mfn in both the source and the destination
domain then we might skip the check in the m2p_override but we don't
care because by luck we got the correct value anyway (m2p(mfn) ==
m2p_override(mfn)).



However I understand that it might be subtle and needs a very long
comment so a simpler alternative would be to explicitly remove the
IDENTITY_FRAME_BIT from the check:

if ((get_phys_to_machine(pfn) & ~IDENTITY_FRAME_BIT) != mfn)

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