From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965111AbbJHRWi (ORCPT ); Thu, 8 Oct 2015 13:22:38 -0400 Received: from eu-smtp-delivery-143.mimecast.com ([207.82.80.143]:55202 "EHLO eu-smtp-delivery-143.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S964961AbbJHRWh convert rfc822-to-8bit (ORCPT ); Thu, 8 Oct 2015 13:22:37 -0400 Subject: Re: [PATCH 03/15] arm64: Introduce helpers for page table levels To: Christoffer Dall , Marc Zyngier 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> Cc: linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org, ard.biesheuvel@linaro.org, Catalin Marinas , Will Deacon , linux-kernel@vger.kernel.org, kvmarm@lists.cs.columbia.edu From: "Suzuki K. Poulose" Message-ID: <5616A65A.50003@arm.com> Date: Thu, 8 Oct 2015 18:22:34 +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: <20151008144546.GD20936@cbox> X-OriginalArrivalTime: 08 Oct 2015 17:22:34.0417 (UTC) FILETIME=[EBBA0A10:01D101ED] X-MC-Unique: Hhg5COy1QYC6gSD8MCxtNQ-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 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