From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Suzuki K. Poulose" Subject: Re: [PATCH 03/15] arm64: Introduce helpers for page table levels Date: Thu, 8 Oct 2015 18:22:34 +0100 Message-ID: <5616A65A.50003@arm.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=WINDOWS-1252; format=flowed Content-Transfer-Encoding: 8BIT 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 To: Christoffer Dall , Marc Zyngier Return-path: In-Reply-To: <20151008144546.GD20936@cbox> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.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