Reading /dev/mem by dd

["Anton D. Kachalov" <mouse@mayc.ru>]

Hello everyone!

I've found strange behavior of reading /dev/mem:

for i in 0 1 2; do
  echo $i
  dd if=/dev/mem of=/dev/null skip=$((6+$i)) bs=$((0x20000000)) count=1
done

On some systems with Supermicro X8DTU I've got several messages during 
first 512Mb starting from 0xc000_0000:

   "BUG: soft lockup - CPU#xx stuck for 61s!"

On other systems with the sameboard I've stuck without any errors at 
last 10Mb before 0x1_0000_0000. Local APIC access?

On E5440 (Dell PowerEdge 1950) I've just got several:
APIC error on CPU3: 00(80)
APIC error on CPU3: 80(80)
...
APIC error on CPU3: 80(80)
That looks like wrong register access.

[E5530] $ cat /proc/iomem
00000000-0000ffff : reserved
00010000-0009dbff : System RAM
0009dc00-0009ffff : reserved
000c0000-000cffff : pnp 00:0e
000e0000-000fffff : reserved
00100000-bf77ffff : System RAM
  00200000-006a37de : Kernel code
  006a37df-0091a57f : Kernel data
  00a68000-00b7cbaf : Kernel bss
bf78e000-bf78ffff : reserved
bf790000-bf79dfff : ACPI Tables
bf79e000-bf7cffff : ACPI Non-volatile Storage
bf7d0000-bf7dffff : reserved
bf7ec000-bfffffff : reserved
e0000000-efffffff : PCI MMCONFIG 0
  e0000000-efffffff : reserved
    e0000000-efffffff : pnp 00:0d
f9000000-f9ffffff : PCI Bus 0000:07
  f9000000-f9ffffff : 0000:07:01.0
faeda000-faeda0ff : 0000:00:1f.3
faedc000-faedc3ff : 0000:00:1d.7
  faedc000-faedc3ff : ehci_hcd
faede000-faede3ff : 0000:00:1a.7
  faede000-faede3ff : ehci_hcd
faee0000-faee3fff : 0000:00:16.7
faee4000-faee7fff : 0000:00:16.6
faee8000-faeebfff : 0000:00:16.5
faeec000-faeeffff : 0000:00:16.4
faef0000-faef3fff : 0000:00:16.3
faef4000-faef7fff : 0000:00:16.2
faef8000-faefbfff : 0000:00:16.1
faefc000-faefffff : 0000:00:16.0
faf00000-faffffff : PCI Bus 0000:01
  faf00000-faf1ffff : 0000:01:00.1
  faf3c000-faf3ffff : 0000:01:00.1
    faf3c000-faf3ffff : igb
  faf40000-faf5ffff : 0000:01:00.1
    faf40000-faf5ffff : igb
  faf60000-faf7ffff : 0000:01:00.1
    faf60000-faf7ffff : igb
  faf80000-faf9ffff : 0000:01:00.0
  fafbc000-fafbffff : 0000:01:00.0
    fafbc000-fafbffff : igb
  fafc0000-fafdffff : 0000:01:00.0
    fafc0000-fafdffff : igb
  fafe0000-faffffff : 0000:01:00.0
    fafe0000-faffffff : igb
fb000000-fbefffff : PCI Bus 0000:07
  fb000000-fb7fffff : 0000:07:01.0
  fbefc000-fbefffff : 0000:07:01.0
fec00000-fec00fff : IOAPIC 0
  fec00000-fec00fff : pnp 00:0c
fed00000-fed003ff : HPET 0
fed1c000-fed1ffff : pnp 00:01
  fed1c000-fed1ffff : pnp 00:0a
fed20000-fed3ffff : pnp 00:0a
fed40000-fed8ffff : pnp 00:0a
fee00000-fee00fff : Local APIC
  fee00000-fee00fff : reserved
    fee00000-fee00fff : pnp 00:0c
ffc00000-ffffffff : reserved
100000000-13fffffff : System RAM

[    0.000000] BIOS-provided physical RAM map:
[    0.000000]  BIOS-e820: 0000000000000000 - 000000000009dc00 (usable)
[    0.000000]  BIOS-e820: 000000000009dc00 - 00000000000a0000 (reserved)
[    0.000000]  BIOS-e820: 00000000000e0000 - 0000000000100000 (reserved)
[    0.000000]  BIOS-e820: 0000000000100000 - 00000000bf780000 (usable)
[    0.000000]  BIOS-e820: 00000000bf78e000 - 00000000bf790000 type 9
[    0.000000]  BIOS-e820: 00000000bf790000 - 00000000bf79e000 (ACPI data)
[    0.000000]  BIOS-e820: 00000000bf79e000 - 00000000bf7d0000 (ACPI NVS)
[    0.000000]  BIOS-e820: 00000000bf7d0000 - 00000000bf7e0000 (reserved)
[    0.000000]  BIOS-e820: 00000000bf7ec000 - 00000000c0000000 (reserved)
[    0.000000]  BIOS-e820: 00000000e0000000 - 00000000f0000000 (reserved)
[    0.000000]  BIOS-e820: 00000000fee00000 - 00000000fee01000 (reserved)
[    0.000000]  BIOS-e820: 00000000ffc00000 - 0000000100000000 (reserved)
[    0.000000]  BIOS-e820: 0000000100000000 - 0000000140000000 (usable)

[E5440] $ cat /proc/iomem
00000000-0009ffff : System RAM
  00000000-00000000 : Crash kernel
000a0000-000bffff : Video RAM area
000c0000-000c7fff : Video ROM
000c9000-000c9fff : Adapter ROM
000ca000-000cfbff : Adapter ROM
000d0000-000d1dff : Adapter ROM
000d2000-000d71ff : Adapter ROM
000f0000-000fffff : System ROM
00100000-bfb4ffff : System RAM
  00200000-003e1fbc : Kernel code
  003e1fbd-004bafe7 : Kernel data
bfb50000-bfb65fff : reserved
bfb66000-bfb85bff : ACPI Tables
bfb85c00-bfffffff : reserved
d0000000-d7ffffff : PCI Bus #10
  d0000000-d7ffffff : 0000:10:0d.0
d8000000-d80fffff : PCI Bus #0c
  d8000000-d80fffff : PCI Bus #0d
    d80f0000-d80fffff : 0000:0d:0e.0
      d80f0000-d80fffff : megasas: LSI Logic
e0000000-efffffff : reserved
f2000000-f7ffffff : PCI Bus #04
  f4000000-f7ffffff : PCI Bus #05
    f4000000-f7ffffff : PCI Bus #06
      f4000000-f7ffffff : PCI Bus #07
        f4000000-f5ffffff : 0000:07:00.0
          f4000000-f5ffffff : bnx2
f8000000-fbffffff : PCI Bus #02
  f8000000-fbffffff : PCI Bus #03
    f8000000-f9ffffff : 0000:03:00.0
      f8000000-f9ffffff : bnx2
fc100000-fc2fffff : PCI Bus #10
  fc100000-fc11ffff : 0000:10:0d.0
  fc2d0000-fc2dffff : 0000:10:0d.0
fc300000-fc5fffff : PCI Bus #0c
  fc400000-fc5fffff : PCI Bus #0d
    fc400000-fc407fff : 0000:0d:0e.0
    fc5c0000-fc5dffff : 0000:0d:0e.0
      fc5c0000-fc5dffff : megasas: LSI Logic
fc600000-fc8fffff : PCI Bus #01
  fc7e0000-fc7effff : 0000:01:00.0
  fc7fc000-fc7fffff : 0000:01:00.0
  fc800000-fc8fffff : 0000:01:00.0
fc900000-fc9003ff : 0000:00:1d.7
  fc900000-fc9003ff : ehci_hcd
fe000000-ffffffff : reserved
100000000-43fffffff : System RAM

HP - ProLiant DL160G6 says:
http://www.pixsup.com/uploads/7134cd3e33.png

Rgds,
Anton