From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id A0615C433F5 for ; Wed, 5 Oct 2022 14:08:20 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 047376B0072; Wed, 5 Oct 2022 10:08:20 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id F389C6B0073; Wed, 5 Oct 2022 10:08:19 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id DD9C86B0074; Wed, 5 Oct 2022 10:08:19 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0012.hostedemail.com [216.40.44.12]) by kanga.kvack.org (Postfix) with ESMTP id CB0656B0072 for ; Wed, 5 Oct 2022 10:08:19 -0400 (EDT) Received: from smtpin07.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay06.hostedemail.com (Postfix) with ESMTP id 9BBD7A6FA1 for ; Wed, 5 Oct 2022 14:08:19 +0000 (UTC) X-FDA: 79987075518.07.6553E66 Received: from mga03.intel.com (mga03.intel.com [134.134.136.65]) by imf19.hostedemail.com (Postfix) with ESMTP id B5CC51A001D for ; Wed, 5 Oct 2022 14:08:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1664978898; x=1696514898; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=x4Mh62nAwfxY8Pita2q5xFpK6rXI3Y7xCTC5ET9xbaw=; b=XcTwy1mMYhYXdFfO2Qgg+9lKnVqZndVhZ18vjTduP9uH7FtsHn98Cg6L GnaTrTqb6cMAgsAZauttLwNoWrDtmNGZc7EjN1Oo9R0+DGamG75tnG8Il U31OTZ5GQiag9xEgMmehLNK8WCmur0jMQPlpfP/wRqs5SZa4jFkh09S7S wTyerpFd0n8nJbwpKwCPdLOa+xcOJtNyoiWLEF3upOn2hPi4XxkZ0r6QK yrMsXJBO8alhlXpT3/nUKnHVp6kJs0uWTumOcov9a88Rmk4XaXCilcmus Hc78CZ6+XJd+Pg3G0xUraWPRjKf8ap+aIYdnmF4yCMt/ZILl+0CPKlq2g A==; X-IronPort-AV: E=McAfee;i="6500,9779,10490"; a="304733826" X-IronPort-AV: E=Sophos;i="5.95,159,1661842800"; d="scan'208";a="304733826" Received: from orsmga006.jf.intel.com ([10.7.209.51]) by orsmga103.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 05 Oct 2022 07:08:17 -0700 X-IronPort-AV: E=McAfee;i="6500,9779,10490"; a="602018913" X-IronPort-AV: E=Sophos;i="5.95,159,1661842800"; d="scan'208";a="602018913" Received: from mghender-mobl.amr.corp.intel.com (HELO [10.209.6.185]) ([10.209.6.185]) by orsmga006-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 05 Oct 2022 07:08:16 -0700 Message-ID: <715095e6-6c4e-62dd-6631-b096db2cd92c@intel.com> Date: Wed, 5 Oct 2022 07:08:15 -0700 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Re: [PATCH v2 10/39] x86/mm: Introduce _PAGE_COW Content-Language: en-US To: Andrew Cooper , Rick Edgecombe , "x86@kernel.org" , "H . Peter Anvin" , Thomas Gleixner , Ingo Molnar , "linux-kernel@vger.kernel.org" , "linux-doc@vger.kernel.org" , "linux-mm@kvack.org" , "linux-arch@vger.kernel.org" , "linux-api@vger.kernel.org" , Arnd Bergmann , Andy Lutomirski , Balbir Singh , Borislav Petkov , Cyrill Gorcunov , Dave Hansen , Eugene Syromiatnikov , Florian Weimer , "H . J . Lu" , Jann Horn , Jonathan Corbet , Kees Cook , Mike Kravetz , Nadav Amit , Oleg Nesterov , Pavel Machek , Peter Zijlstra , Randy Dunlap , "Ravi V . Shankar" , Weijiang Yang , "Kirill A . Shutemov" , "joao.moreira@intel.com" , John Allen , "kcc@google.com" , "eranian@google.com" , "rppt@kernel.org" , "jamorris@linux.microsoft.com" , "dethoma@microsoft.com" Cc: Yu-cheng Yu References: <20220929222936.14584-1-rick.p.edgecombe@intel.com> <20220929222936.14584-11-rick.p.edgecombe@intel.com> <54cdad9f-b810-7966-5928-9320d970a43d@citrix.com> From: Dave Hansen In-Reply-To: <54cdad9f-b810-7966-5928-9320d970a43d@citrix.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1664978899; a=rsa-sha256; cv=none; b=WDzSz0CAS9Tb8pITJvf0UQzWQP6s1f/TKmIB39O8DIpS2Jnl2mscFuAGsWb8VjrwjZKGEL JkyGjaYZCKZHDiqfRQcJX0ryRG8Viptlkl9VVLeBFvy6vznZOpUw30HBftv7fdeCyfP46i hJktLx8GiF+c6NI+xAYPekkBk9Z6NgU= ARC-Authentication-Results: i=1; imf19.hostedemail.com; dkim=none ("invalid DKIM record") header.d=intel.com header.s=Intel header.b=XcTwy1mM; spf=pass (imf19.hostedemail.com: domain of dave.hansen@intel.com designates 134.134.136.65 as permitted sender) smtp.mailfrom=dave.hansen@intel.com; dmarc=pass (policy=none) header.from=intel.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1664978899; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=U6rQ+V8s4Ma2dnDA8dcILWuzvd0kekdLDV4R3adXWW8=; b=7M3UIU28xC1/ca1NF0uGAHHm0FdPnzPo3k74kVfQZKu8ooHVsH2UMFJgYyhTvxlhGNwl0V rFzXeMJ79zN3pTEdwr5TqBNoBtuVTJJQH3/f29Ya9r6V9HxG9n0VB8D8mNs0YfiGQ+W+fO pXuiD4P8lmo0W4PU1kCCAA7Rv19wMc8= X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: B5CC51A001D X-Rspam-User: Authentication-Results: imf19.hostedemail.com; dkim=none ("invalid DKIM record") header.d=intel.com header.s=Intel header.b=XcTwy1mM; spf=pass (imf19.hostedemail.com: domain of dave.hansen@intel.com designates 134.134.136.65 as permitted sender) smtp.mailfrom=dave.hansen@intel.com; dmarc=pass (policy=none) header.from=intel.com X-Stat-Signature: zccobiaqsgpkw484139eimwhts4wgbkh X-HE-Tag: 1664978898-788824 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 10/4/22 19:17, Andrew Cooper wrote: > On 29/09/2022 23:29, Rick Edgecombe wrote: >> From: Yu-cheng Yu >> >> There is essentially no room left in the x86 hardware PTEs on some OSes >> (not Linux). That left the hardware architects looking for a way to >> represent a new memory type (shadow stack) within the existing bits. >> They chose to repurpose a lightly-used state: Write=0,Dirty=1. > How does "Some OSes have a greater dependence on software available bits > in PTEs than Linux" sound? > >> The reason it's lightly used is that Dirty=1 is normally set _before_ a >> write. A write with a Write=0 PTE would typically only generate a fault, >> not set Dirty=1. Hardware can (rarely) both set Write=1 *and* generate the >> fault, resulting in a Dirty=0,Write=1 PTE. Hardware which supports shadow >> stacks will no longer exhibit this oddity. > Again, an interesting anecdote but not salient information here. As much as I like the sound of my own voice (and anecdotes), I agree that this is a bit oblique for the patch. Maybe this anecdote should get banished elsewhere. The changelog here could definitely get to the point faster.