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 X-Spam-Level: X-Spam-Status: No, score=-5.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5FFC9C4742C for ; Mon, 16 Nov 2020 19:35:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0F1C820825 for ; Mon, 16 Nov 2020 19:35:17 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=oracle.com header.i=@oracle.com header.b="QB14Jbfp" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729669AbgKPTfQ (ORCPT ); Mon, 16 Nov 2020 14:35:16 -0500 Received: from aserp2120.oracle.com ([141.146.126.78]:34854 "EHLO aserp2120.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726849AbgKPTfP (ORCPT ); Mon, 16 Nov 2020 14:35:15 -0500 Received: from pps.filterd (aserp2120.oracle.com [127.0.0.1]) by aserp2120.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 0AGJKV8x095517; Mon, 16 Nov 2020 19:34:48 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oracle.com; h=subject : to : cc : references : from : message-id : date : mime-version : in-reply-to : content-type : content-transfer-encoding; s=corp-2020-01-29; bh=RWeINXuiLje/N3MSlb65Y0oEcBF4evaveY/U8g1ActI=; b=QB14JbfpPqCawSI/ocC2Jv0oOzSkjuoDIDW8IEjQQjpC2ULn7W+3Mp/zV+mTuBxj5DXo HH3fmRLRtob9LRLIJH3wAVkPf1Ch3htRH8f43LmQGx0T8GynShMWQ3CtlvJrOR/THv5M 1FnQRU1vok7d2vfoaf2OicP5ZCI8Xi1ESOYpm/zPU86dpPewD31VbvpU+k7ZFfHWvC6D h/oKKQ4YygnNpbtlFe+oTgDxrnqdKqctWkjEp/Dg8VWea2L5dMS9mUxbKfitRGR4vT6y 6mIdGwyPMLMr26wzbWzRnfkL0aH9XuQZhhrhS7se5Q1WSpRxEUgd+X6p9r3oUQEya/G/ DA== Received: from userp3030.oracle.com (userp3030.oracle.com [156.151.31.80]) by aserp2120.oracle.com with ESMTP id 34t76kpvk5-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Mon, 16 Nov 2020 19:34:48 +0000 Received: from pps.filterd (userp3030.oracle.com [127.0.0.1]) by userp3030.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 0AGJKUgF170672; Mon, 16 Nov 2020 19:34:47 GMT Received: from aserv0121.oracle.com (aserv0121.oracle.com [141.146.126.235]) by userp3030.oracle.com with ESMTP id 34ts5v3jun-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 16 Nov 2020 19:34:47 +0000 Received: from abhmp0001.oracle.com (abhmp0001.oracle.com [141.146.116.7]) by aserv0121.oracle.com (8.14.4/8.13.8) with ESMTP id 0AGJYh0T001026; Mon, 16 Nov 2020 19:34:43 GMT Received: from localhost.localdomain (/92.157.91.83) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Mon, 16 Nov 2020 11:34:43 -0800 Subject: Re: [RFC][PATCH v2 12/21] x86/pti: Use PTI stack instead of trampoline stack To: Andy Lutomirski Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , X86 ML , Dave Hansen , Peter Zijlstra , LKML , Tom Lendacky , Joerg Roedel , Konrad Rzeszutek Wilk , jan.setjeeilers@oracle.com, Junaid Shahid , oweisse@google.com, Mike Rapoport , Alexander Graf , mgross@linux.intel.com, kuzuno@gmail.com References: <20201116144757.1920077-1-alexandre.chartre@oracle.com> <20201116144757.1920077-13-alexandre.chartre@oracle.com> From: Alexandre Chartre Message-ID: <93d1f346-7513-069f-dcd9-24f2ea009145@oracle.com> Date: Mon, 16 Nov 2020 20:37:03 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9807 signatures=668682 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 adultscore=0 spamscore=0 phishscore=0 suspectscore=0 mlxscore=0 malwarescore=0 bulkscore=0 mlxlogscore=999 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2011160115 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9807 signatures=668682 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 phishscore=0 adultscore=0 priorityscore=1501 bulkscore=0 clxscore=1015 mlxlogscore=999 malwarescore=0 mlxscore=0 spamscore=0 lowpriorityscore=0 impostorscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2011160115 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/16/20 7:34 PM, Andy Lutomirski wrote: > On Mon, Nov 16, 2020 at 10:10 AM Alexandre Chartre > wrote: >> >> >> On 11/16/20 5:57 PM, Andy Lutomirski wrote: >>> On Mon, Nov 16, 2020 at 6:47 AM Alexandre Chartre >>> wrote: >>>> >>>> When entering the kernel from userland, use the per-task PTI stack >>>> instead of the per-cpu trampoline stack. Like the trampoline stack, >>>> the PTI stack is mapped both in the kernel and in the user page-table. >>>> Using a per-task stack which is mapped into the kernel and the user >>>> page-table instead of a per-cpu stack will allow executing more code >>>> before switching to the kernel stack and to the kernel page-table. >>> >>> Why? >> >> When executing more code in the kernel, we are likely to reach a point >> where we need to sleep while we are using the user page-table, so we need >> to be using a per-thread stack. >> >>> I can't immediately evaluate how nasty the page table setup is because >>> it's not in this patch. >> >> The page-table is the regular page-table as introduced by PTI. It is just >> augmented with a few additional mapping which are in patch 11 (x86/pti: >> Extend PTI user mappings). >> >>> But AFAICS the only thing that this enables is sleeping with user pagetables. >> >> That's precisely the point, it allows to sleep with the user page-table. >> >>> Do we really need to do that? >> >> Actually, probably not with this particular patchset, because I do the page-table >> switch at the very beginning and end of the C handler. I had some code where I >> moved the page-table switch deeper in the kernel handler where you definitively >> can sleep (for example, if you switch back to the user page-table before >> exit_to_user_mode_prepare()). >> >> So a first step should probably be to not introduce the per-task PTI trampoline stack, >> and stick with the existing trampoline stack. The per-task PTI trampoline stack can >> be introduced later when the page-table switch is moved deeper in the C handler and >> we can effectively sleep while using the user page-table. > > Seems reasonable. > > Where is the code that allocates and frees these stacks hiding? I > think I should at least read it. Stacks are allocated/freed with the task stack, this code is unchanged (see alloc_thread_stack_node()). The trick is that I have doubled the THREAD_SIZE (patch 8 "x86/pti: Introduce per-task PTI trampoline stack"). Half the stack is a used as the kernel stack (mapped only in the kernel page-table), the other half is used as the PTI stack (mapped in the kernel and user page-table). The mapping to the user page-table is done in mm_map_task() in fork.c (patch 11 "x86/pti: Extend PTI user mappings"). alex.