xen: avoid another early crash of memory limited dom0

Commit b1c9f169047b ("xen: split counting of extra memory pages...")
introduced an error when dom0 was started with limited memory occurring
only on some hardware.

The problem arises in case dom0 is started with initial memory and
maximum memory being the same. The kernel must be configured without
CONFIG_XEN_BALLOON_MEMORY_HOTPLUG for the problem to happen. If all
of this is true and the E820 map of the machine is sparse (some areas
are not covered) then the machine might crash early in the boot
process.

An example E820 map triggering the problem looks like this:

[    0.000000] e820: BIOS-provided physical RAM map:
[    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009d7ff] usable
[    0.000000] BIOS-e820: [mem 0x000000000009d800-0x000000000009ffff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000000e0000-0x00000000000fffff] reserved
[    0.000000] BIOS-e820: [mem 0x0000000000100000-0x00000000cf7fafff] usable
[    0.000000] BIOS-e820: [mem 0x00000000cf7fb000-0x00000000cf95ffff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000cf960000-0x00000000cfb62fff] ACPI NVS
[    0.000000] BIOS-e820: [mem 0x00000000cfb63000-0x00000000cfd14fff] usable
[    0.000000] BIOS-e820: [mem 0x00000000cfd15000-0x00000000cfd61fff] ACPI NVS
[    0.000000] BIOS-e820: [mem 0x00000000cfd62000-0x00000000cfd6cfff] ACPI data
[    0.000000] BIOS-e820: [mem 0x00000000cfd6d000-0x00000000cfd6ffff] ACPI NVS
[    0.000000] BIOS-e820: [mem 0x00000000cfd70000-0x00000000cfd70fff] usable
[    0.000000] BIOS-e820: [mem 0x00000000cfd71000-0x00000000cfea8fff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000cfea9000-0x00000000cfeb9fff] ACPI NVS
[    0.000000] BIOS-e820: [mem 0x00000000cfeba000-0x00000000cfecafff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000cfecb000-0x00000000cfecbfff] ACPI NVS
[    0.000000] BIOS-e820: [mem 0x00000000cfecc000-0x00000000cfedbfff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000cfedc000-0x00000000cfedcfff] ACPI NVS
[    0.000000] BIOS-e820: [mem 0x00000000cfedd000-0x00000000cfeddfff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000cfede000-0x00000000cfee3fff] ACPI NVS
[    0.000000] BIOS-e820: [mem 0x00000000cfee4000-0x00000000cfef6fff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000cfef7000-0x00000000cfefffff] usable
[    0.000000] BIOS-e820: [mem 0x00000000e0000000-0x00000000efffffff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000fec00000-0x00000000fec00fff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000fec10000-0x00000000fec10fff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000fed00000-0x00000000fed00fff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000fed40000-0x00000000fed44fff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000fed61000-0x00000000fed70fff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000fed80000-0x00000000fed8ffff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000ff000000-0x00000000ffffffff] reserved
[    0.000000] BIOS-e820: [mem 0x0000000100001000-0x000000020effffff] usable

In this case the area a0000-dffff isn't present in the map. This will
confuse the memory setup of the domain when remapping the memory from
such holes to populated areas.

To avoid the problem the accounting of to be remapped memory has to
count such holes in the E820 map as well.

Reported-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

arch/x86/xen/setup.c

@@ -593,20 +593,27 @@ static void __init xen_ignore_unusable(void)
 static unsigned long __init xen_count_remap_pages(unsigned long max_pfn)
 {
 	unsigned long extra = 0;
+	unsigned long start_pfn, end_pfn;
 	const struct e820entry *entry = xen_e820_map;
 	int i;
 
+	end_pfn = 0;
 	for (i = 0; i < xen_e820_map_entries; i++, entry++) {
-		unsigned long start_pfn = PFN_DOWN(entry->addr);
-		unsigned long end_pfn = PFN_UP(entry->addr + entry->size);
+		start_pfn = PFN_DOWN(entry->addr);
+		/* Adjacent regions on non-page boundaries handling! */
+		end_pfn = min(end_pfn, start_pfn);
 
 		if (start_pfn >= max_pfn)
-			break;
-		if (entry->type == E820_RAM)
-			continue;
-		if (end_pfn >= max_pfn)
-			end_pfn = max_pfn;
-		extra += end_pfn - start_pfn;
+			return extra + max_pfn - end_pfn;
+
+		/* Add any holes in map to result. */
+		extra += start_pfn - end_pfn;
+
+		end_pfn = PFN_UP(entry->addr + entry->size);
+		end_pfn = min(end_pfn, max_pfn);
+
+		if (entry->type != E820_RAM)
+			extra += end_pfn - start_pfn;
 	}
 
 	return extra;