On 07/10/15 10:26, Marc Zyngier wrote:
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.
Thanks Marc for pitching in. That explains it better.
I'd vote for the DIV_ROUND_UP(), which will make things a lot more readable.
Sure, I can change that.
Suzuki
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