Why system can not show all the memory on RHEL6 when using AMD processors.

Issue

RHEL6.0 can not recognize all the memory when using AMD processors, the system will have similar below info in dmesg.

BIOS-provided physical RAM map:
 BIOS-e820: 0000000000000000 - 000000000009f000 (usable)
 BIOS-e820: 000000000009f000 - 00000000000a0000 (reserved)
 BIOS-e820: 00000000000f0000 - 0000000000100000 (reserved)
 BIOS-e820: 0000000000100000 - 00000000bde1c000 (usable)
 BIOS-e820: 00000000bde1c000 - 00000000bde4c000 (ACPI data)
 BIOS-e820: 00000000bde4c000 - 00000000bde4d000 (usable)
 BIOS-e820: 00000000bde4d000 - 00000000d0000000 (reserved)
 BIOS-e820: 00000000fec00000 - 00000000fee10000 (reserved)
 BIOS-e820: 00000000ff800000 - 0000000100000000 (reserved)
 BIOS-e820: 0000000100000000 - 000000103ffff000 (usable)
DMI 2.7 present.
e820 update range: 0000000000000000 - 0000000000001000 (usable) ==> (reserved)
e820 remove range: 00000000000a0000 - 0000000000100000 (usable)
last_pfn = 0x103ffff max_arch_pfn = 0x400000000
MTRR default type: uncachable
MTRR fixed ranges enabled:
  00000-9FFFF write-back
  A0000-BFFFF uncachable
  C0000-FFFFF write-back
MTRR variable ranges enabled:
  0 base 000000000000 mask FFFF80000000 write-back
  1 base 000080000000 mask FFFFC0000000 write-back
  2 disabled
  3 disabled
  4 disabled
  5 disabled
  6 base 0000FF800000 mask FFFFFF800000 write-protect
  7 disabled
x86 PAT enabled: cpu 0, old 0x7040600070406, new 0x7010600070106
original variable MTRRs
reg 0, base: 0GB, range: 2GB, type WB
reg 1, base: 2GB, range: 1GB, type WB
reg 6, base: 4088MB, range: 8MB, type WP
total RAM covered: 3072M
Found optimal setting for mtrr clean up
 gran_size: 64K     chunk_size: 64K     num_reg: 2      lose cover RAM: 0G
New variable MTRRs
reg 0, base: 0GB, range: 2GB, type WB
reg 1, base: 2GB, range: 1GB, type WB
e820 update range: 00000000c0000000 - 000000103ffff000 (usable) ==> (reserved)
WARNING: BIOS bug: CPU MTRRs don't cover all of memory, losing 62463MB of RAM.
update e820 for mtrr
modified physical RAM map:
 modified: 0000000000000000 - 0000000000001000 (reserved)
 modified: 0000000000001000 - 000000000009f000 (usable)
 modified: 000000000009f000 - 00000000000a0000 (reserved)
 modified: 00000000000f0000 - 0000000000100000 (reserved)
 modified: 0000000000100000 - 00000000bde1c000 (usable)
 modified: 00000000bde1c000 - 00000000bde4c000 (ACPI data)
 modified: 00000000bde4c000 - 00000000bde4d000 (usable)
 modified: 00000000bde4d000 - 00000000d0000000 (reserved)
 modified: 00000000fec00000 - 00000000fee10000 (reserved)
 modified: 00000000ff800000 - 000000103ffff000 (reserved)