xen/m2p: Check whether the MFN has IDENTITY_FRAME bit set..

If there is no proper PFN value in the M2P for the MFN
(so we get 0xFFFFF.. or 0x55555, or 0x0), we should
consult the M2P override to see if there is an entry for this.
[Note: we also consult the M2P override if the MFN
is past our machine_to_phys size].

We consult the P2M with the PFN. In case the returned
MFN is one of the special values: 0xFFF.., 0x5555
(which signify that the MFN can be either "missing" or it
belongs to DOMID_IO) or the p2m(m2p(mfn)) != mfn, we check
the M2P override. If we fail the M2P override check, we reset
the PFN value to INVALID_P2M_ENTRY.

Next we try to find the MFN in the P2M using the MFN
value (not the PFN value) and if found, we know
that this MFN is an identity value and return it as so.

Otherwise we have exhausted all the posibilities and we
return the PFN, which at this stage can either be a real
PFN value found in the machine_to_phys.. array, or
INVALID_P2M_ENTRY value.

[v1: Added Review-by tag]
Reviewed-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/include/asm/xen/page.h

@@ -81,6 +81,7 @@ static inline int phys_to_machine_mapping_valid(unsigned long pfn)
 static inline unsigned long mfn_to_pfn(unsigned long mfn)
 {
 	unsigned long pfn;
+	int ret = 0;
 
 	if (xen_feature(XENFEAT_auto_translated_physmap))
 		return mfn;
@@ -95,15 +96,29 @@ static inline unsigned long mfn_to_pfn(unsigned long mfn)
 	 * In such cases it doesn't matter what we return (we return garbage),
 	 * but we must handle the fault without crashing!
 	 */
-	__get_user(pfn, &machine_to_phys_mapping[mfn]);
+	ret = __get_user(pfn, &machine_to_phys_mapping[mfn]);
 try_override:
+	/* ret might be < 0 if there are no entries in the m2p for mfn */
+	if (ret < 0)
+		pfn = ~0;
+	else if (get_phys_to_machine(pfn) != 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.
+		 *
+		 * m2p_find_override_pfn returns ~0 if it doesn't find anything.
 		 */
-	if (get_phys_to_machine(pfn) != mfn)
-		pfn = m2p_find_override_pfn(mfn, pfn);
+		pfn = m2p_find_override_pfn(mfn, ~0);
+
+	/* 
+	 * pfn is ~0 if there are no entries in the m2p for mfn or if the
+	 * entry doesn't map back to the mfn and m2p_override doesn't have a
+	 * valid entry for it.
+	 */
+	if (pfn == ~0 &&
+			get_phys_to_machine(mfn) == IDENTITY_FRAME(mfn))
+		pfn = mfn;
 
 	return pfn;
 }