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 30F04C2D0E4 for ; Tue, 17 Nov 2020 15:07:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BFD9E24199 for ; Tue, 17 Nov 2020 15:07:53 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=oracle.com header.i=@oracle.com header.b="E/JjIl19" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387589AbgKQPHw (ORCPT ); Tue, 17 Nov 2020 10:07:52 -0500 Received: from aserp2130.oracle.com ([141.146.126.79]:58232 "EHLO aserp2130.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729056AbgKQPHw (ORCPT ); Tue, 17 Nov 2020 10:07:52 -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 0AHF4EZ9086169; Tue, 17 Nov 2020 15:07:06 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=LJ5Gpontmz7S2CLUM9pY0583Z0BZHf4rwl89j0dY5Ks=; b=E/JjIl19XJ1QYmf5EKM2C/uB+KlTg062hrw4Lbgh0cBxqzW7u4u5c2Wk63Oyv2QohAfj S+9FgGfZtbQObtiyqMF3VAV5smF5DkFFNBVqhK1Bzfj2wtTzf6Bz7m3ur+ilh/BQPG6h 7Mh/nNqAVf3KPOeZsA60neXF2pyqyGagVkSq2WRbNpMSm8ZeJU1vORH14J6GHMrzQZdc 9Z4Le0T7MUs0iEbxFWAUsSMcw+36Kk6vsSirEYp8C4X6TKywdyjrZ86Qt4GgMTc44vG8 dygCVIQaAuEYGj4Odeimlmi+8xYx2DqMmWnXtyH/SnKUowCtfHIEUQDGLoOjBeZVNMDe ZA== Received: from aserp3030.oracle.com (aserp3030.oracle.com [141.146.126.71]) by aserp2130.oracle.com with ESMTP id 34t4rau1xp-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Tue, 17 Nov 2020 15:07:06 +0000 Received: from pps.filterd (aserp3030.oracle.com [127.0.0.1]) by aserp3030.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 0AHExaRb162222; Tue, 17 Nov 2020 15:07:06 GMT Received: from aserv0121.oracle.com (aserv0121.oracle.com [141.146.126.235]) by aserp3030.oracle.com with ESMTP id 34usptg84k-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 17 Nov 2020 15:07:05 +0000 Received: from abhmp0010.oracle.com (abhmp0010.oracle.com [141.146.116.16]) by aserv0121.oracle.com (8.14.4/8.13.8) with ESMTP id 0AHF6t5U006040; Tue, 17 Nov 2020 15:07:00 GMT Received: from linux.home (/92.157.91.83) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Tue, 17 Nov 2020 07:06:54 -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: Tue, 17 Nov 2020 16:09:15 +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 mlxlogscore=999 malwarescore=0 mlxscore=0 bulkscore=0 suspectscore=0 adultscore=0 spamscore=0 phishscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2011170112 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-2011170112 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. > I finally remember why I have introduced a per-task PTI trampoline stack right now: that's to be able to move the CR3 switch anywhere in the C handler. To do so, we need a per-task stack to enter (and return) from the C handler as the handler can potentially go to sleep. Without a per-task trampoline stack, we would be limited to call the switch CR3 functions from the assembly entry code before and after calling the C function handler (also called from assembly). alex.