x86: check mptable physptr with max_low_pfn on 32bit

Impact: fix early crash on LinuxBIOS systems

Kevin O'Connor reported that Coreboot aka LinuxBIOS tries to put
mptable somewhere very high, well above max_low_pfn (below which
BIOSes generally put the mptable), causing a panic.

The BIOS will probably be changed to be compatible with older
Linus versions, but nevertheless the MP-spec does not forbid
an MP-table in arbitrary system RAM, so make sure it all
works even if the table is in an unexpected place.

Check physptr with max_low_pfn * PAGE_SIZE.
Reported-by: Kevin O'Connor <kevin@koconnor.net>
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Stefan Reinauer <stepan@coresystems.de>
Cc: coreboot@coreboot.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/mpparse.c

@@ -710,14 +710,23 @@ static int __init smp_scan_config(unsigned long base, unsigned long length,
 				 * of physical memory; so that simply reserving
 				 * PAGE_SIZE from mpf->physptr yields BUG()
 				 * in reserve_bootmem.
+				 * also need to make sure physptr is below than
+				 * max_low_pfn
+				 * we don't need reserve the area above max_low_pfn
 				 */
 				unsigned long end = max_low_pfn * PAGE_SIZE;
+
+				if (mpf->physptr < end) {
 					if (mpf->physptr + size > end)
 						size = end - mpf->physptr;
-#endif
 					reserve_bootmem_generic(mpf->physptr, size,
 							BOOTMEM_DEFAULT);
 				}
+#else
+				reserve_bootmem_generic(mpf->physptr, size,
+						BOOTMEM_DEFAULT);
+#endif
+			}
 
 			return 1;
 		}