xen: switch to linear virtual mapped sparse p2m list

At start of the day the Xen hypervisor presents a contiguous mfn list
to a pv-domain. In order to support sparse memory this mfn list is
accessed via a three level p2m tree built early in the boot process.
Whenever the system needs the mfn associated with a pfn this tree is
used to find the mfn.

Instead of using a software walked tree for accessing a specific mfn
list entry this patch is creating a virtual address area for the
entire possible mfn list including memory holes. The holes are
covered by mapping a pre-defined  page consisting only of "invalid
mfn" entries. Access to a mfn entry is possible by just using the
virtual base address of the mfn list and the pfn as index into that
list. This speeds up the (hot) path of determining the mfn of a
pfn.

Kernel build on a Dell Latitude E6440 (2 cores, HT) in 64 bit Dom0
showed following improvements:

Elapsed time: 32:50 ->  32:35
System:       18:07 ->  17:47
User:        104:00 -> 103:30

Tested with following configurations:
- 64 bit dom0, 8GB RAM
- 64 bit dom0, 128 GB RAM, PCI-area above 4 GB
- 32 bit domU, 512 MB, 8 GB, 43 GB (more wouldn't work even without
                                    the patch)
- 32 bit domU, ballooning up and down
- 32 bit domU, save and restore
- 32 bit domU with PCI passthrough
- 64 bit domU, 8 GB, 2049 MB, 5000 MB
- 64 bit domU, ballooning up and down
- 64 bit domU, save and restore
- 64 bit domU with PCI passthrough

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

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

@@ -65,13 +65,25 @@ extern unsigned long m2p_find_override_pfn(unsigned long mfn, unsigned long pfn)
  *   bits (identity or foreign) are set.
  * - __pfn_to_mfn() returns the found entry of the p2m table. A possibly set
  *   identity or foreign indicator will be still set. __pfn_to_mfn() is
- *   encapsulating get_phys_to_machine().
- * - get_phys_to_machine() is to be called by __pfn_to_mfn() only to allow
- *   for future optimizations.
+ *   encapsulating get_phys_to_machine() which is called in special cases only.
+ * - get_phys_to_machine() is to be called by __pfn_to_mfn() only in special
+ *   cases needing an extended handling.
  */
 static inline unsigned long __pfn_to_mfn(unsigned long pfn)
 {
-	return get_phys_to_machine(pfn);
+	unsigned long mfn;
+
+	if (pfn < xen_p2m_size)
+		mfn = xen_p2m_addr[pfn];
+	else if (unlikely(pfn < xen_max_p2m_pfn))
+		return get_phys_to_machine(pfn);
+	else
+		return IDENTITY_FRAME(pfn);
+
+	if (unlikely(mfn == INVALID_P2M_ENTRY))
+		return get_phys_to_machine(pfn);
+
+	return mfn;
 }
 
 static inline unsigned long pfn_to_mfn(unsigned long pfn)