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 F3F33C2D0A3 for ; Mon, 16 Nov 2020 18:10:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9E4EE20B80 for ; Mon, 16 Nov 2020 18:10:41 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=oracle.com header.i=@oracle.com header.b="zxnyyoC+" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730958AbgKPSKk (ORCPT ); Mon, 16 Nov 2020 13:10:40 -0500 Received: from aserp2130.oracle.com ([141.146.126.79]:45966 "EHLO aserp2130.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730379AbgKPSKj (ORCPT ); Mon, 16 Nov 2020 13:10:39 -0500 Received: from pps.filterd (aserp2130.oracle.com [127.0.0.1]) by aserp2130.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 0AGIABEY108138; Mon, 16 Nov 2020 18:10:11 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=+uC7Y26/uXFxZx87Xoqi883QbKzE1gBD6NslXGIAf+Y=; b=zxnyyoC+JA5RXg7U6ZwW8wDj2Cdv8wSgEOkfGZOuII7TPueriQ0qzkm852jgHTHsQIQn PaqX48MHIGzGVjUCDKFUYHvBOFlAKLzTqO+9lDvHbjHsUBiZ69HGcwCWEEpeMEjHdk1E hQo86i1VQPf+xyUtWbVSoJ8ek2kdCpfgwBXokDKuuluY0hCkGpBoSJ72Yn/2fsr9usYz lkE8it1SCMKaVJUYVAhR6NlFe/yPKZ/wcmpjtcu12I5aCv6S1YShdS1MTs5gH1sXfhZv P9+0+6RK2oodWKrndduxbFFiwsTN/HrS6pIp+Ln7xAv8bIEIkz1oXkFLJIyc4suq70Nx Tw== Received: from userp3030.oracle.com (userp3030.oracle.com [156.151.31.80]) by aserp2130.oracle.com with ESMTP id 34t4rapk5e-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Mon, 16 Nov 2020 18:10:11 +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 0AGI0Wmh123257; Mon, 16 Nov 2020 18:08:06 GMT Received: from userv0122.oracle.com (userv0122.oracle.com [156.151.31.75]) by userp3030.oracle.com with ESMTP id 34ts5v0n04-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 16 Nov 2020 18:08:06 +0000 Received: from abhmp0020.oracle.com (abhmp0020.oracle.com [141.146.116.26]) by userv0122.oracle.com (8.14.4/8.14.4) with ESMTP id 0AGI81kh009056; Mon, 16 Nov 2020 18:08:02 GMT Received: from localhost.localdomain (/92.157.91.83) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Mon, 16 Nov 2020 10:08:01 -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: Date: Mon, 16 Nov 2020 19:10:22 +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-2011160106 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9807 signatures=668682 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 bulkscore=0 clxscore=1015 malwarescore=0 impostorscore=0 lowpriorityscore=0 priorityscore=1501 mlxlogscore=999 adultscore=0 phishscore=0 suspectscore=0 spamscore=0 mlxscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2011160107 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. alex.