From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934172AbbJIJWy (ORCPT ); Fri, 9 Oct 2015 05:22:54 -0400 Received: from eu-smtp-delivery-143.mimecast.com ([146.101.78.143]:9702 "EHLO eu-smtp-delivery-143.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756300AbbJIJWZ convert rfc822-to-8bit (ORCPT ); Fri, 9 Oct 2015 05:22:25 -0400 Subject: Re: [PATCH 03/15] arm64: Introduce helpers for page table levels To: Catalin Marinas References: <1442331684-28818-1-git-send-email-suzuki.poulose@arm.com> <1442331684-28818-4-git-send-email-suzuki.poulose@arm.com> <20151007082652.GM9011@cbox> <5614E536.9040007@arm.com> <20151008144546.GD20936@cbox> <5616A65A.50003@arm.com> <20151008172856.GQ17192@e104818-lin.cambridge.arm.com> Cc: Christoffer Dall , Marc Zyngier , kvm@vger.kernel.org, ard.biesheuvel@linaro.org, Will Deacon , linux-kernel@vger.kernel.org, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org From: "Suzuki K. Poulose" Message-ID: <5617874F.5010104@arm.com> Date: Fri, 9 Oct 2015 10:22:23 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: <20151008172856.GQ17192@e104818-lin.cambridge.arm.com> X-OriginalArrivalTime: 09 Oct 2015 09:22:23.0314 (UTC) FILETIME=[01625B20:01D10274] X-MC-Unique: Lx9K_AQqQtaoxmGzc71_lw-1 Content-Type: text/plain; charset=WINDOWS-1252; format=flowed Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/10/15 18:28, Catalin Marinas wrote: > On Thu, Oct 08, 2015 at 06:22:34PM +0100, Suzuki K. Poulose wrote: >> On 08/10/15 15:45, Christoffer Dall wrote: >>> On Wed, Oct 07, 2015 at 10:26:14AM +0100, Marc Zyngier wrote: >>>> 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. > > Or just keep the original magic formula and add the DIV_ROUND_UP one in > a comment. > OK, will keep proper documentation with the cryptic formula ;) Suzuki