On 08/10/15 15:45, Christoffer Dall wrote:
On Wed, Oct 07, 2015 at 10:26:14AM +0100, Marc Zyngier wrote:
On 07/10/15 09:26, Christoffer Dall wrote:
Hi Suzuki,
I just had a chat with Catalin, who did shed some light on this.
It all has to do with rounding up. What you would like to have here is:
#define ARM64_HW_PGTABLE_LEVELS(va_bits) DIV_ROUND_UP(va_bits - PAGE_SHIFT, PAGE_SHIFT - 3)
where (va_bits - PAGE_SHIFT) is the total number of bits we deal
with during a page table walk, and (PAGE_SHIFT - 3) is the number
of bits we deal with per level.
The clue is in how DIV_ROUND_UP is written:
#define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
which gives you Suzuki's magic formula.
I'd vote for the DIV_ROUND_UP(), which will make things a lot more readable.
Thanks for the explanation, I vote for DIV_ROUND_UP too.
Btw, DIV_ROUND_UP is defined in linux/kernel.h, including which in the required
headers breaks the build. I could add the definition of the same locally.
You can stash this away for a cryptic interview question ;)
;)
Suzuki
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