From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753000AbbJONO3 (ORCPT ); Thu, 15 Oct 2015 09:14:29 -0400 Received: from eu-smtp-delivery-143.mimecast.com ([146.101.78.143]:21537 "EHLO eu-smtp-delivery-143.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752941AbbJONO1 convert rfc822-to-8bit (ORCPT ); Thu, 15 Oct 2015 09:14:27 -0400 Subject: Re: [PATCHv3 03/11] arm64: Introduce helpers for page table levels To: Mark Rutland , Christoffer Dall References: <1444821634-1689-1-git-send-email-suzuki.poulose@arm.com> <1444821634-1689-4-git-send-email-suzuki.poulose@arm.com> <20151015113735.GB21930@cbox> <20151015124451.GI8825@leverpostej> Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, catalin.marinas@arm.com, will.deacon@arm.com, steve.capper@linaro.org, marc.zyngier@arm.com, ard.biesheuvel@linaro.org From: "Suzuki K. Poulose" Message-ID: <561FA6AE.6090707@arm.com> Date: Thu, 15 Oct 2015 14:14:22 +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: <20151015124451.GI8825@leverpostej> X-OriginalArrivalTime: 15 Oct 2015 13:14:22.0720 (UTC) FILETIME=[6879A800:01D1074B] X-MC-Unique: bowjPbVlSGODvSpyCOQeSQ-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 15/10/15 13:44, Mark Rutland wrote: > On Thu, Oct 15, 2015 at 01:37:35PM +0200, Christoffer Dall wrote: >> On Wed, Oct 14, 2015 at 12:20:26PM +0100, Suzuki K. Poulose wrote: >>> Introduce helpers for finding the number of page table >>> levels required for a given VA width, shift for a particular >>> page table level. >>> +/* >>> + * Size mapped by an entry at level n >>> + * We map PAGE_SHIFT - 3 at all levels, except the PAGE_SHIFT bits at the last level >>> + */ >>> +#define ARM64_HW_PGTABLE_LEVEL_SHIFT(n) ((PAGE_SHIFT - 3) * (4 - (n)) + 3) >> >> I feel like I'm partially failing the interview question again, in that >> I don't fully understand the '+ 3' in the end? > > The last level handles PAGE_SHIFT bits (the bits from the VA that are > the same in the PA). We only accounted for (PAGE_SHIFT - 3) bits at each > level when multiplying, so we add those 3 missing bits back at the end. > Something like : /* * Size mapped by an entry at level n * We map PAGE_SHIFT - 3 at all levels, except the last, where we map PAGE_SHIFT bits. * The maximum number of levels supported by the architecture is 4. Hence at starting * at level n, we hanve (4 - n) levels of translation. So, the total number of bits * mapped by an entry at level n is : * * ((4 - n) - 1) * (PAGE_SHIFT - 3) + PAGE_SHIFT * * Rearranging it a bit we get : * (4 - n) * (PAGE_SHIFT - 3) + 3 */ Or we could use the formula without rearranging. Thanks Suzuki