xen/x86: Workaround 64-bit hypervisor and 32-bit initial domain.

If a 64-bit hypervisor is booted with a 32-bit initial domain,
the hypervisor deals with the initial domain as "compat" and
does some extra adjustments (like pagetables are 4 bytes instead
of 8). It also adjusts the xen_start_info->pt_base incorrectly.

When booted with a 32-bit hypervisor (32-bit initial domain):
..
(XEN)  Start info:    cf831000->cf83147c
(XEN)  Page tables:   cf832000->cf8b5000
..
[    0.000000] PT: cf832000 (f832000)
[    0.000000] Reserving PT: f832000->f8b5000

And with a 64-bit hypervisor:
(XEN)  Start info:    00000000cf831000->00000000cf8314b4
(XEN)  Page tables:   00000000cf832000->00000000cf8b6000

[    0.000000] PT: cf834000 (f834000)
[    0.000000] Reserving PT: f834000->f8b8000

To deal with this, we keep keep track of the highest physical
address we have reserved via memblock_reserve. If that address
does not overlap with pt_base, we have a gap which we reserve.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/enlighten.c

@@ -1002,19 +1002,24 @@ static int xen_write_msr_safe(unsigned int msr, unsigned low, unsigned high)
  * If the MFN is not in the m2p (provided to us by the hypervisor) this
  * function won't do anything. In practice this means that the XenBus
  * MFN won't be available for the initial domain. */
-static void __init xen_reserve_mfn(unsigned long mfn)
+static unsigned long __init xen_reserve_mfn(unsigned long mfn)
 {
-	unsigned long pfn;
+	unsigned long pfn, end_pfn = 0;
 
 	if (!mfn)
-		return;
+		return end_pfn;
+
 	pfn = mfn_to_pfn(mfn);
-	if (phys_to_machine_mapping_valid(pfn))
-		memblock_reserve(PFN_PHYS(pfn), PAGE_SIZE);
+	if (phys_to_machine_mapping_valid(pfn)) {
+		end_pfn = PFN_PHYS(pfn) + PAGE_SIZE;
+		memblock_reserve(PFN_PHYS(pfn), end_pfn);
+	}
+	return end_pfn;
 }
 static void __init xen_reserve_internals(void)
 {
 	unsigned long size;
+	unsigned long last_phys = 0;
 
 	if (!xen_pv_domain())
 		return;
@@ -1022,12 +1027,13 @@ static void __init xen_reserve_internals(void)
 	/* xen_start_info does not exist in the M2P, hence can't use
 	 * xen_reserve_mfn. */
 	memblock_reserve(__pa(xen_start_info), PAGE_SIZE);
+	last_phys = __pa(xen_start_info) + PAGE_SIZE;
 
-	xen_reserve_mfn(PFN_DOWN(xen_start_info->shared_info));
-	xen_reserve_mfn(xen_start_info->store_mfn);
+	last_phys = max(xen_reserve_mfn(PFN_DOWN(xen_start_info->shared_info)), last_phys);
+	last_phys = max(xen_reserve_mfn(xen_start_info->store_mfn), last_phys);
 
 	if (!xen_initial_domain())
-		xen_reserve_mfn(xen_start_info->console.domU.mfn);
+		last_phys = max(xen_reserve_mfn(xen_start_info->console.domU.mfn), last_phys);
 
 	if (xen_feature(XENFEAT_auto_translated_physmap))
 		return;
@@ -1043,8 +1049,14 @@ static void __init xen_reserve_internals(void)
 	 * a lot (and call memblock_reserve for each PAGE), so lets just use
 	 * the easy way and reserve it wholesale. */
 	memblock_reserve(__pa(xen_start_info->mfn_list), size);
-
+	last_phys = max(__pa(xen_start_info->mfn_list) + size, last_phys);
 	/* The pagetables are reserved in mmu.c */
+
+	/* Under 64-bit hypervisor with a 32-bit domain, the hypervisor
+	 * offsets the pt_base by two pages. Hence the reservation that is done
+	 * in mmu.c misses two pages. We correct it here if we detect this. */
+	if (last_phys < __pa(xen_start_info->pt_base))
+		memblock_reserve(last_phys, __pa(xen_start_info->pt_base) - last_phys);
 }
 void xen_setup_shared_info(void)
 {