We tack on the IDENTITY_FRAME_BIT on all PFNs which have been
elected during the E820 parsing to be identity mapping.

This way, in case a freak occurs such that pfn_to_mfn(pfn)==pfn
for non-identity PFNs we can guard against that and not consider
it a 1-1 mapping.

Also this will allows us to leverage this and tack on _PAGE_IOMAP
on any page that has IDENTITY_FRAME_BIT set (see:
"xen/mmu: Set _PAGE_IOMAP if PFN is in identity P2M.").

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 arch/x86/include/asm/xen/page.h |    6 +++++-
 arch/x86/xen/mmu.c              |   11 ++++++++---
 arch/x86/xen/setup.c            |    2 +-
 3 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/xen/page.h b/arch/x86/include/asm/xen/page.h
index be671f6..a1e4ce8 100644
--- a/arch/x86/include/asm/xen/page.h
+++ b/arch/x86/include/asm/xen/page.h
@@ -30,6 +30,7 @@ typedef struct xpaddr {
 /**** MACHINE <-> PHYSICAL CONVERSION MACROS ****/
 #define INVALID_P2M_ENTRY	(~0UL)
 #define FOREIGN_FRAME_BIT	(1UL<<31)
+#define IDENTITY_FRAME_BIT	(1UL<<30)
 #define FOREIGN_FRAME(m)	((m) | FOREIGN_FRAME_BIT)
 
 /* Maximum amount of memory we can handle in a domain in pages */
@@ -52,9 +53,12 @@ static inline unsigned long pfn_to_mfn(unsigned long pfn)
 
 	mfn = get_phys_to_machine(pfn);
 
-	if (mfn != INVALID_P2M_ENTRY)
+	if (mfn != INVALID_P2M_ENTRY) {
 		mfn &= ~FOREIGN_FRAME_BIT;
 
+		if (mfn & IDENTITY_FRAME_BIT)
+			mfn &= ~IDENTITY_FRAME_BIT;
+	}
 	return mfn;
 }
 
diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index d98bd43..d470435 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -425,10 +425,10 @@ unsigned long get_phys_to_machine(unsigned long pfn)
 	 * would be wrong.
 	 */
 	if (p2m_top[topidx] == p2m_mid_identity)
-		return pfn;
+		return pfn | IDENTITY_FRAME_BIT;
 
 	if (p2m_top[topidx][mididx] == p2m_identity)
-		return pfn;
+		return pfn | IDENTITY_FRAME_BIT;
 
 	return p2m_top[topidx][mididx][idx];
 ...
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I don't think the inner-if buys us anything here and the whole thing is
equivalent to:
	if (mfn != INVALID_P2M_ENTRY)
		mfn &= ~(FOREIGN_FRAME_BIT|IDENTITY_FRAME_BIT);

Not sure if the FOREIGN_FRAME_BIT|IDENTITY_FRAME_BIT construct gets

It's probably worth defining IDENTITY_FRAME(m) in the pattern of
FOREIGN_FRAME(m).

Ian.

--

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<nods>
--

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