[Crash-utility] using crash for ARM

[Mika Westerberg]

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?