[Crash-utility] [PATH v4 2/2] arm64: add 4-level translation

[AKASHI Takahiro]

On Wed, Jun 01, 2016 at 10:32:56AM -0400, Dave Anderson wrote:
> 
> ----- Original Message -----
> > On Tue, May 31, 2016 at 03:30:44PM -0400, Dave Anderson wrote:
> > > 
> > > This patch looks good -- if it wasn't layered on top of the KASLR patch,
> > > I would check it into github.
> > 
> > This patch is almost independent from KASLR support, except that
> > adding new flags, VM_L4_4K and NEW_VMEMMAP, is conflicting.
> > 
> > When you merge it, please make a change.
> > 
> > -Takahiro AKASHI
> 
> OK, I see that now.  But upon further inspection, I've got another question
> about this piece:
> 
>  + * 4-levels / 4K pages
>  + * 48-bit VA
>  + */
>  +#define PTRS_PER_PGD_L4_4K   ((1UL) << (48 - 39))
>  +#define PTRS_PER_PUD_L4_4K   (512)
>  +#define PTRS_PER_PMD_L4_4K   (512)
>  +#define PTRS_PER_PTE_L4_4K   (512)
>  +#define PGDIR_SHIFT_L4_4K    (39)
>  +#define PGDIR_SIZE_L4_4K     ((1UL) << PGDIR_SHIFT_L4_4K)
>  +#define PGDIR_MASK_L4_4K     (~(PGDIR_SIZE_L4_4K-1))
>  +#define PUD_SHIFT_L4_4K      (30)
>  +#define PUD_SIZE_L4_4K       ((1UL) << PUD_SHIFT_L4_4K)
>  +#define PUD_MASK_L4_4K       (~(PUD_SIZE_L4_4K-1))
>  +#define PMD_SHIFT_L4_4K      (21)
>  +#define PMD_SIZE_L4_4K       (1UL << PMD_SHIFT_L4_4K)
>  +#define PMD_MASK_L4_4K       (~(PMD_SIZE_L4_4K-1))
>  +
>  +#define PGDIR_SIZE      (1UL << ((48 - 39) + 3))
>  +#define PGDIR_MASK      (~(PGDIR_SIZE - 1))
>  +#define PGDIR_OFFSET(X) (((ulong)(X)) & (PGDIR_SIZE - 1))
>  +
> 
> You have defined PGDIR_SIZE_L4_4K and PGDIR_MASK_L4_4K, but they
> are not used in arm64.c:
> 
>   $ grep -e PGDIR_SIZE_L4_4K -e PGDIR_MASK_L4_4K arm64.c
>   $

It is kinda copy-and-paste from L3_4K.
PGDIR_MASK_L3_4K/PMD_SIZE_L3_4K/PMD_MASK_L3_4K are not used neither.

> Instead you define PGDIR_SIZE and PGDIR_MASK without the _L4_4K qualifiers,
> and use them and PGDIR_OFFSET() in arm64.c.
> 
> Preferably convention should be followed such that the _L4_4K versions would
> be used, but I see that PGDIR_SIZE_L4_4K is not equal to PGDIR_SIZE, and that
> PGDIR_MASK_L4_4K is not equal to PGDIR_MASK.  What's going on there?

As you can see, those macros are used to calculate pgd_val and pud_base.
I'd like to use the same formula as in the other level of tables,
but the size and mask can be varied depending on a value of VA_BITS
for 4-level translation.
For 48-bit VA, luckily or not, PAGEOFFSET() would work instead of
PGDIR_OFFSET(), and likewise machdep->pagemask instead of PGDIR_MASK,
but we should not use them.
That is why I define the macros as
    #define PGDIR_SIZE  (1UL << ((48 - 39) + 3))

But I will agree that we'd better give them different names to avoid
any confusion.
Any suggestions?

Thanks,
-Takahiro AKASHI

> Dave
> 
> 
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-- 
Thanks,
-Takahiro AKASHI