[PATCHv3 03/11] arm64: Introduce helpers for page table levels
Suzuki K. Poulose
Suzuki.Poulose at arm.com
Thu Oct 15 06:14:22 PDT 2015
On 15/10/15 13:44, Mark Rutland wrote:
> On Thu, Oct 15, 2015 at 01:37:35PM +0200, Christoffer Dall wrote:
>> On Wed, Oct 14, 2015 at 12:20:26PM +0100, Suzuki K. Poulose wrote:
>>> Introduce helpers for finding the number of page table
>>> levels required for a given VA width, shift for a particular
>>> page table level.
>>> + * Size mapped by an entry at level n
>>> + * We map PAGE_SHIFT - 3 at all levels, except the PAGE_SHIFT bits at the last level
>>> + */
>>> +#define ARM64_HW_PGTABLE_LEVEL_SHIFT(n) ((PAGE_SHIFT - 3) * (4 - (n)) + 3)
>> I feel like I'm partially failing the interview question again, in that
>> I don't fully understand the '+ 3' in the end?
> The last level handles PAGE_SHIFT bits (the bits from the VA that are
> the same in the PA). We only accounted for (PAGE_SHIFT - 3) bits at each
> level when multiplying, so we add those 3 missing bits back at the end.
Something like :
* Size mapped by an entry at level n
* We map PAGE_SHIFT - 3 at all levels, except the last, where we map PAGE_SHIFT bits.
* The maximum number of levels supported by the architecture is 4. Hence at starting
* at level n, we hanve (4 - n) levels of translation. So, the total number of bits
* mapped by an entry at level n is :
* ((4 - n) - 1) * (PAGE_SHIFT - 3) + PAGE_SHIFT
* Rearranging it a bit we get :
* (4 - n) * (PAGE_SHIFT - 3) + 3
Or we could use the formula without rearranging.
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