linux-kernel - Re: [PATCH 03/15] arm64: Introduce helpers for page table levels

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Date:	Wed, 7 Oct 2015 10:51:16 +0100
From:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
To:	Christoffer Dall <christoffer.dall@...aro.org>
Cc:	linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org,
	ard.biesheuvel@...aro.org, Marc.Zyngier@....com,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	linux-kernel@...r.kernel.org, kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCH 03/15] arm64: Introduce helpers for page table levels

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@....com>
>>
>> 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@...aro.org>
>> Cc: Mark Rutland <mark.rutland@....com>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Will Deacon <will.deacon@....com>
>> Signed-off-by: Suzuki K. Poulose <suzuki.poulose@....com>
>> Reviewed-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>> Tested-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>> ---
>>   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) ?

As mentioned, I will change it to DIV_ROUND_UP() as suggested by Marc.

>
>> +#define ARM64_HW_PGTABLE_LEVEL_SHIFT(level) \
>> +		((PAGE_SHIFT - 3) * (level) + 3)
>> +
>
> While this change is clearly correct, if you can explain the math here
> in a comment as well, that would be helpful.

Sure, will add a comment to that effect.

Suzuki

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