On 07/10/15 09:26, Christoffer Dall wrote: > Hi Suzuki, > > On Tue, Sep 15, 2015 at 04:41:12PM +0100, Suzuki K. Poulose wrote: >> From: "Suzuki K. Poulose" <suzuki.poulose@xxxxxxx> >> >> Introduce helpers for finding the number of page table >> levels required for a given VA width, shift for a particular >> page table level. >> >> Convert the existing users to the new helpers. More users >> to follow. >> >> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> >> Cc: Mark Rutland <mark.rutland@xxxxxxx> >> Cc: Catalin Marinas <catalin.marinas@xxxxxxx> >> Cc: Will Deacon <will.deacon@xxxxxxx> >> Signed-off-by: Suzuki K. Poulose <suzuki.poulose@xxxxxxx> >> Reviewed-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> >> Tested-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> >> --- >> arch/arm64/include/asm/pgtable-hwdef.h | 15 ++++++++++++--- >> 1 file changed, 12 insertions(+), 3 deletions(-) >> >> diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h >> index 24154b0..ce18389 100644 >> --- a/arch/arm64/include/asm/pgtable-hwdef.h >> +++ b/arch/arm64/include/asm/pgtable-hwdef.h >> @@ -16,13 +16,21 @@ >> #ifndef __ASM_PGTABLE_HWDEF_H >> #define __ASM_PGTABLE_HWDEF_H >> >> +/* >> + * Number of page-table levels required to address 'va_bits' wide >> + * address, without section mapping >> + */ >> +#define ARM64_HW_PGTABLE_LEVELS(va_bits) (((va_bits) - 4) / (PAGE_SHIFT - 3)) > > I don't understand the '(va_bits) - 4' here, can you explain it (and add a > comment to that effect) ? 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. M. -- Jazz is not dead. It just smells funny... -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
