From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753356AbbJGJsk (ORCPT ); Wed, 7 Oct 2015 05:48:40 -0400 Received: from eu-smtp-delivery-143.mimecast.com ([207.82.80.143]:57449 "EHLO eu-smtp-delivery-143.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751592AbbJGJsi convert rfc822-to-8bit (ORCPT ); Wed, 7 Oct 2015 05:48:38 -0400 Subject: Re: [PATCH 03/15] arm64: Introduce helpers for page table levels To: Marc Zyngier , Christoffer Dall 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> 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: <5614EA73.5060807@arm.com> Date: Wed, 7 Oct 2015 10:48:35 +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: <5614E536.9040007@arm.com> X-OriginalArrivalTime: 07 Oct 2015 09:48:35.0558 (UTC) FILETIME=[55B04060:01D100E5] X-MC-Unique: BTHFSRF6TcqH8NW9_l1a8g-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 07/10/15 10:26, Marc Zyngier wrote: > On 07/10/15 09:26, Christoffer Dall wrote: >> Hi Suzuki, >> >> On Tue, Sep 15, 2015 at 04:41:12PM +0100, Suzuki K. Poulose wrote: >>> From: "Suzuki K. Poulose" >>> >>> Introduce helpers for finding the number of page table >>> levels required for a given VA width, shift for a particular >>> page table level. >>> >>> Convert the existing users to the new helpers. More users >>> to follow. >>> >>> Cc: Ard Biesheuvel >>> Cc: Mark Rutland >>> Cc: Catalin Marinas >>> Cc: Will Deacon >>> Signed-off-by: Suzuki K. Poulose >>> Reviewed-by: Ard Biesheuvel >>> Tested-by: Ard Biesheuvel >>> --- >>> arch/arm64/include/asm/pgtable-hwdef.h | 15 ++++++++++++--- >>> 1 file changed, 12 insertions(+), 3 deletions(-) >>> >>> diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h >>> index 24154b0..ce18389 100644 >>> --- a/arch/arm64/include/asm/pgtable-hwdef.h >>> +++ b/arch/arm64/include/asm/pgtable-hwdef.h >>> @@ -16,13 +16,21 @@ >>> #ifndef __ASM_PGTABLE_HWDEF_H >>> #define __ASM_PGTABLE_HWDEF_H >>> >>> +/* >>> + * Number of page-table levels required to address 'va_bits' wide >>> + * address, without section mapping >>> + */ >>> +#define ARM64_HW_PGTABLE_LEVELS(va_bits) (((va_bits) - 4) / (PAGE_SHIFT - 3)) >> >> I don't understand the '(va_bits) - 4' here, can you explain it (and add a >> comment to that effect) ? > > 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. Thanks Marc for pitching in. That explains it better. > > I'd vote for the DIV_ROUND_UP(), which will make things a lot more readable. Sure, I can change that. Suzuki