Re: unaligned access

"Kevin D. Kissell" <kevink@xxxxxxxx> · Thu, 22 Feb 2007 16:06:57 +0100

Default behavior in MIPS is to silently fix up and 
emulate.  A MIPS-specific
system call (sys_sysmips with the command argument 
of MIPS_FIXADE
and a parameter agument of zero) allows for this to be overridden, so that 
such accesses will be fatal.  It looks as 
if there was once support to log the events 

to syslog, independently of whether or not they were fixed up, but it doesn't look to me 

as if that still works in 2.6.x kernels.

       Regards,

    Kevin K.

  ----- Original Message ----- 
  From: 
  sathesh babu 
  To: Rajat Jain 
  Cc: linux-mips@xxxxxxxxxxxxxx 
  Sent: Thursday, February 22, 2007 10:18 
  AM
  Subject: Re: unaligned access

  Thanks Rajan.

  In case of arm processor, the alignment trap behavior can be changed by 
  simply echo a number into  /proc/sys/debug/alignment 

  bit             
  behavior when 
  set
---             
  -----------------
  0               
  A user process performing an unaligned memory 
  access

  will cause the kernel to print a message 
  indicating

  process name, pid, pc, instruction, address, and 
  the

  fault code.
  1               
  The kernel will attempt to fix up the user 
  process

  performing the unaligned access.  This is of 
  course

  slow (think about the floating point emulator) 
  and

  not recommended for production use.
  2               
  The kernel will send a SIGBUS signal to the user 
  process

  performing the unaligned access.

I would like to know  Is there 
  similar type of implimentation avalilable for MIPS processor in linux-2.6.12 
  kernel to view or log the unaligned access addresses and corresponding 
  processor ID.

  Regards,
  Sathesh

Rajat Jain <rajat.noida.india@xxxxxxxxx> 
  wrote:
  On 
    2/22/07, sathesh babu wrote:
> 
    Hi,
> I have ported linux-2.6.12 kernel on MIPS processor.I would like 
    to
> print the warning messges whenenver kernel or user code try to 
    access
> unaligned address ( including proceor ID ).
> Is there 
    any configuration option avaliable in the kernel to view
> the 
    unaligned address?

Ummm ... not sure about MIPS, but in i386, 
    exception 17 is raised for
every unaligned access. alignment_check() is 
    invoked for every 
  such
access.

Regards,

Rajat

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