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From: "Suzuki K. Poulose" <Suzuki.Poulose@arm.com>
To: Christoffer Dall <christoffer.dall@linaro.org>,
	Marc Zyngier <marc.zyngier@arm.com>
Cc: linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org,
	ard.biesheuvel@linaro.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	linux-kernel@vger.kernel.org, kvmarm@lists.cs.columbia.edu
Subject: Re: [PATCH 03/15] arm64: Introduce helpers for page table levels
Date: Thu, 8 Oct 2015 18:22:34 +0100	[thread overview]
Message-ID: <5616A65A.50003@arm.com> (raw)
In-Reply-To: <20151008144546.GD20936@cbox>

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


  reply	other threads:[~2015-10-08 17:22 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-09-15 15:41 [PATCHv2 00/15] arm64: 16K translation granule support Suzuki K. Poulose
2015-09-15 15:41 ` [PATCH 01/15] arm64: Move swapper pagetable definitions Suzuki K. Poulose
2015-09-15 15:41 ` [PATCH 02/15] arm64: Handle section maps for swapper/idmap Suzuki K. Poulose
2015-09-15 15:41 ` [PATCH 03/15] arm64: Introduce helpers for page table levels Suzuki K. Poulose
2015-10-07  8:26   ` Christoffer Dall
2015-10-07  9:26     ` Marc Zyngier
2015-10-07  9:48       ` Suzuki K. Poulose
2015-10-08 14:45       ` Christoffer Dall
2015-10-08 17:22         ` Suzuki K. Poulose [this message]
2015-10-08 17:28           ` Catalin Marinas
2015-10-09  9:22             ` Suzuki K. Poulose
2015-10-07  9:51     ` Suzuki K. Poulose
2015-09-15 15:41 ` [PATCH 04/15] arm64: Calculate size for idmap_pg_dir at compile time Suzuki K. Poulose
2015-09-15 15:41 ` [PATCH 05/15] arm64: Handle 4 level page table for swapper Suzuki K. Poulose
2015-09-15 15:41 ` [PATCH 06/15] arm64: Clean config usages for page size Suzuki K. Poulose
2015-09-15 15:41 ` [PATCH 07/15] arm64: Kconfig: Fix help text about AArch32 support with 64K pages Suzuki K. Poulose
2015-09-15 15:41 ` [PATCH 08/15] arm64: Check for selected granule support Suzuki K. Poulose
2015-09-15 15:41 ` [PATCH 09/15] arm64: Add page size to the kernel image header Suzuki K. Poulose
2015-10-02 15:49   ` Catalin Marinas
2015-10-02 16:31     ` Catalin Marinas
2015-10-02 16:50       ` Marc Zyngier
2015-10-05 15:43         ` Christoffer Dall
2015-10-05 13:02     ` Suzuki K. Poulose
2015-10-05 13:22       ` Ard Biesheuvel
2015-10-10 17:22   ` Christoffer Dall
2015-09-15 15:41 ` [PATCH 10/15] arm64: kvm: Fix {V}TCR_EL2_TG0 mask Suzuki K. Poulose
2015-10-08 15:17   ` Christoffer Dall
2015-09-15 15:41 ` [PATCH 11/15] arm64: Cleanup VTCR_EL2 computation Suzuki K. Poulose
2015-10-07 10:11   ` Marc Zyngier
2015-10-07 10:23     ` Suzuki K. Poulose
2015-10-10 17:22   ` Christoffer Dall
2015-09-15 15:41 ` [PATCH 12/15] arm: kvm: Move fake PGD handling to arch specific files Suzuki K. Poulose
2015-10-07 10:23   ` Marc Zyngier
2015-10-10 17:22     ` Christoffer Dall
2015-09-15 15:41 ` [PATCH 13/15] arm64: kvm: Rewrite fake pgd handling Suzuki K. Poulose
2015-10-07 11:13   ` Marc Zyngier
2015-10-07 12:21     ` Suzuki K. Poulose
2015-10-10 14:52   ` Christoffer Dall
2015-10-12  9:55     ` Suzuki K. Poulose
2015-10-13 15:39       ` Christoffer Dall
2015-10-13 16:04         ` Suzuki K. Poulose
2015-09-15 15:41 ` [PATCH 14/15] arm64: Add 16K page size support Suzuki K. Poulose
2015-09-15 15:41 ` [PATCH 15/15] arm64: 36 bit VA Suzuki K. Poulose

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