[Crash-utility] using crash for ARM

[paawan oza]

My comment for the fix would be something like below.

/* The unity-mapped region is mapped using 1MB pages, hence 1-level translation 

    if bit 20 is set, we are 1MB apart physically, hence we move the page_dir

    in case bit 20 is set. */

if (bit(vaddr,20))
    page_dir = page_dir + 1;

does it make sense ?

Regards,
Oza.



________________________________
 From: Mika Westerberg <mika.westerberg at iki.fi>
To: Dave Anderson <anderson at redhat.com> 
Cc: paawan oza <paawan1982 at yahoo.com>; "Discussion list for crash utility usage, maintenance and development" <crash-utility at redhat.com>; Thomas Fänge <thomas.fange at sonymobile.com>; jan karlsson <jan.karlsson at sonymobile.com> 
Sent: Thursday, 4 October 2012 11:56 AM
Subject: Re: [Crash-utility] using crash for ARM
 
On Wed, Oct 03, 2012 at 09:45:51AM -0400, Dave Anderson wrote:
> 
> Right -- it looks to be a bug, presuming that ARM is using 1MB pages
> for the unity-mapped region:
>  
>  crash> vtop c0000000 | grep PAGE:
>   PAGE:    11000  (1MB)
>  crash> vtop c0100000 | grep PAGE:
>   PAGE:    11000  (1MB)
>  crash> vtop c0200000 | grep PAGE:
>   PAGE:   211000  (1MB)
>  crash> vtop c0300000 | grep PAGE:
>   PAGE:   211000  (1MB)
>  crash> vtop c0400000 | grep PAGE:
>   PAGE:   411000  (1MB)
>  crash> vtop c0500000 | grep PAGE:
>   PAGE:   411000  (1MB)
>  crash>

The unity-mapped region is mapped using 1MB pages. However, we actually have
(when using the Linux ARM 2-level translation scheme):

see arch/arm/include/asm/pgtable-2level.h:

#define PMD_SHIFT               21
#define PGDIR_SHIFT             21

#define PTRS_PER_PGD            2048

So we have 2048 entries in a PGD instead of 4096 making a PGD entry an array
of "two pointers".

Anyway as you and Paawan suggested it looks like a bug - we always use the
first entry instead of the second given that bit 20 is set in the virtual
address.

Paawan, your fix looks sane to me but can you add a small comment describing
why this is done?
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