From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751125AbaKEXIJ (ORCPT ); Wed, 5 Nov 2014 18:08:09 -0500 Received: from mga09.intel.com ([134.134.136.24]:38625 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750867AbaKEXIE (ORCPT ); Wed, 5 Nov 2014 18:08:04 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.07,322,1413270000"; d="scan'208";a="632251008" Message-ID: <545AADCC.5030102@intel.com> Date: Wed, 05 Nov 2014 15:07:56 -0800 From: Dave Hansen User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0 MIME-Version: 1.0 To: Andi Kleen CC: Masanari Iida , Jonathan Corbet , "linux-kernel@vger.kernel.org" , linux-mm@kvack.org, lcapitulino@redhat.com Subject: Re: [PATCH] Documentation: vm: Add 1GB large page support information References: <1414771317-5721-1-git-send-email-standby24x7@gmail.com> <5457C6EA.3080809@intel.com> <545A42C4.6070908@intel.com> <871tphtftg.fsf@tassilo.jf.intel.com> In-Reply-To: <871tphtftg.fsf@tassilo.jf.intel.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/05/2014 02:58 PM, Andi Kleen wrote: >>> >> I understand that there are some exception cases which doesn't support 1G >>> >> large pages on newer CPUs. >>> >> I like Dave's example, at the same time I would like to add "pdpe1gb flag" in >>> >> the document. >>> >> >>> >> For example, x86 CPUs normally support 4K and 2M (1G if pdpe1gb flag exist). >> > >> > Is 1G supported on CPUs that have pdpe1gb and are running a 32-bit kernel? > No, 1GB pages is a 64bit only feature. This is one sentence in a document that nobody reads, so we're all putting way more brainpower in to this than we should. We can't universally say that "1G if pdpe1gb flag exist" since a 64-bit CPU running a 32-bit kernel doesn't support 1G pages *despite* the presence of pdpe1gb. I think that makes it a pretty crappy thing to put in a document since it's just misleading. We can't spell out all the pitfalls or all the possible combinations, and it's not the place of our stupid documentation to repeat what's in the architecture manuals. For example, x86 CPUs normally support 4K and 2M (1G if architecturally supported).