On Thu, Oct 15, 2015 at 02:14:22PM +0100, Suzuki K. Poulose wrote: > 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
nit: s/hanve/have/ > * 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. > Either way, even I get it now. Thanks for the explanation!! Assuming some version of this goes in: Acked-by: Christoffer Dall <christoffer.d...@linaro.org> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
