MIPS Unaligned Access Question

["Ken Hicks" <hicks@nortel.com>]

Hi,

This is my first post. I hope I'm following correct etiquette.  Here we go....

I'm investigating why an Unaligned Access exception is generated on MIPS from an accesses which are not misaligned.

The issue is that a kernel access two different unmapped addresses results in different exceptions:
Address                Exception
0x0001000000000000:    page fault
0x0010000000000000:    unaligned access

I'm using a Cavium CPU with a custom linux based on 2.6.14 but the code in question hasn't changed widly in more recent kernels.

I have observed this several times, so I have manually recreated the behaviour by intentionally accessing known unmapped addresses.

In this first case, I forced an access to 0x0001000000000000:

Oops in arch/mips/mm/fault.c::do_page_fault, line 232[#15]:
Cpu 5
$ 0   : 0000000000000000 ffffffff81680000 000000000eb5fe30 00000029e2cb9823
$ 4   : 00000000000003e8 00000029e2c02673 000000002cb41780 0000000000000000
$ 8   : 000000000000ed97 0000000000004001 0000000000000001 ffffffff8167d547
$12   : ffffffffffffffff 0000000000000010 ffffffff8167d927 ffffffff81541730
$16   : 0001000000000000 0001000000000000 0000000000000007 ffffffff81619828
$20   : a80000000eb5fe30 a80000000eb5f0a0 a80000000eb5f0a0 ffffffff815a3400
$24   : 0000000000000000 0000000000000030                                
$28   : a80000000eb5c000 a80000000eb5fb30 ffffffffffffff80 ffffffff81101eb0
Hi    : 0000002dc6c00000
Lo    : 0000001e9c578400
epc   : ffffffff81101fd0 kernel_ken+0x2f8/0x310     Tainted: P   
ra    : ffffffff81101eb0 kernel_ken+0x1d8/0x310
Status: 10007fe2    KX SX UX KERNEL EXL
Cause : 4080800c
BadVA : 0001000000000000

In this second case, I forced an access to 0x0010000000000000:

Unhandled kernel unaligned access in
arch/mips/kernel/unaligned.c::emulate_load_store_insn, line 507[#11]:
Cpu 3
$ 0   : 0000000000000000 ffffffff81680000 000000000eb0be30 00000017a7f4fdc1
$ 4   : 00000000000003e8 00000017a7e98c11 000000002cb41780 0000000000000000
$ 8   : 0000000000003272 0000000000004001 0000000000000001 ffffffff8167d547
$12   : ffffffffffffffff 0000000000000010 ffffffff8167d927 ffffffff81541730
$16   : a8000000e62c0980 0010000000000000 0000000000000007 ffffffff81619828
$20   : a80000000eb0be30 000000007fc000e0 000000007fc00190 ffffffff815a3400
$24   : 0000000000000000 0000000000000030                                
$28   : a80000000eb08000 a80000000eb0bb30 0000000000512c54 ffffffff81101eb0
Hi    : 0000002dc6c00000
Lo    : 0000001e9c578400
epc   : ffffffff81101fd0 kernel_ken+0x2f8/0x310     Tainted: P   
ra    : ffffffff81101eb0 kernel_ken+0x1d8/0x310
Status: 10007fe2    KX SX UX KERNEL EXL
Cause : 40808014
BadVA : 0010000000000000

In the second case, the address is not unaligned, but it is reported as an unaligned access error.

Is this behaviour related to some memory mapping?

Here's copy of cat /proc/iomem:
016c0000-08ebffff : System RAM
09010000-0fc0ffff : System RAM
20000000-ffffffff : System RAM
412000000-41fffffff : System RAM
1180000000800-118000000083f : serial
11b0008001000-11b0048001000 : Octeon PCI MEM
  11b0008020000-11b000803ffff : 0000:00:00.0
    11b0008020000-11b000803ffff : e1000
  11b0008040000-11b000805ffff : 0000:00:00.1
    11b0008040000-11b000805ffff : e1000

Is this a bug, or intentional behaviour?

In any case, would anyone be able to explain why the two accesses are reported differently.
I'd just like to understand it.

Thanks,
Ken

Ken Hicks