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

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Date:	Thu, 8 Oct 2015 18:22:34 +0100
From:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
To:	Christoffer Dall <christoffer.dall@...aro.org>,
	Marc Zyngier <marc.zyngier@....com>
Cc:	linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org,
	ard.biesheuvel@...aro.org,
	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 08/10/15 15:45, Christoffer Dall wrote:
> On Wed, Oct 07, 2015 at 10:26:14AM +0100, Marc Zyngier wrote:
>> On 07/10/15 09:26, Christoffer Dall wrote:
>>> Hi Suzuki,
>>>


>> 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.
>>
> Thanks for the explanation, I vote for DIV_ROUND_UP too.

Btw, DIV_ROUND_UP is defined in linux/kernel.h, including which in the required
headers breaks the build. I could add the definition of the same locally.

>
> You can stash this away for a cryptic interview question ;)

;)


Suzuki

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