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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 57D77C433DF for ; Thu, 18 Jun 2020 11:33:05 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 2912C206C3 for ; Thu, 18 Jun 2020 11:33:05 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2912C206C3 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jlsmg-0001is-5J; Thu, 18 Jun 2020 11:32:42 +0000 Received: from all-amaz-eas1.inumbo.com ([34.197.232.57] helo=us1-amaz-eas2.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jlsmf-0001in-4W for xen-devel@lists.xenproject.org; Thu, 18 Jun 2020 11:32:41 +0000 X-Inumbo-ID: 69d7018c-b157-11ea-ba74-12813bfff9fa Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 69d7018c-b157-11ea-ba74-12813bfff9fa; Thu, 18 Jun 2020 11:32:38 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 13478ACA7; Thu, 18 Jun 2020 11:32:37 +0000 (UTC) Subject: Re: [PATCH v2 for-4.14] x86/vmx: use P2M_ALLOC in vmx_load_pdptrs instead of P2M_UNSHARE To: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= References: <3555e16baa9e1909dbefaaab04330694135c9021.1592410065.git.tamas.lengyel@intel.com> <20200618094014.GH735@Air-de-Roger> From: Jan Beulich Message-ID: Date: Thu, 18 Jun 2020 13:32:38 +0200 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 MIME-Version: 1.0 In-Reply-To: <20200618094014.GH735@Air-de-Roger> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: Kevin Tian , Tamas K Lengyel , Wei Liu , Paul Durrant , Andrew Cooper , Jun Nakajima , xen-devel@lists.xenproject.org Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" On 18.06.2020 11:40, Roger Pau Monné wrote: > On Thu, Jun 18, 2020 at 08:30:08AM +0200, Jan Beulich wrote: >> On 17.06.2020 18:19, Tamas K Lengyel wrote: >>> While forking VMs running a small RTOS system (Zephyr) a Xen crash has been >>> observed due to a mm-lock order violation while copying the HVM CPU context >>> from the parent. This issue has been identified to be due to >>> hap_update_paging_modes first getting a lock on the gfn using get_gfn. This >>> call also creates a shared entry in the fork's memory map for the cr3 gfn. The >>> function later calls hap_update_cr3 while holding the paging_lock, which >>> results in the lock-order violation in vmx_load_pdptrs when it tries to unshare >>> the above entry when it grabs the page with the P2M_UNSHARE flag set. >>> >>> Since vmx_load_pdptrs only reads from the page its usage of P2M_UNSHARE was >>> unnecessary to start with. Using P2M_ALLOC is the appropriate flag to ensure >>> the p2m is properly populated and to avoid the lock-order violation we >>> observed. >> >> Using P2M_ALLOC is not going to address the original problem though >> afaict: You may hit the mem_sharing_fork_page() path that way, and >> via nominate_page() => __grab_shared_page() => mem_sharing_page_lock() >> you'd run into a lock order violation again. > > Well, I guess Tamas avoids this because of the get_gfn call in > hap_update_paging_modes will have already populated the entry, so it's > never going to hit the p2m_is_hole check in __get_gfn_type_access. > >> The change is an improvement, so I'd be fine with it going in this >> way, but only as long as the description mentions that there's still >> an open issue here (which may be non-trivial to address). Or perhaps >> combining with your v1 change is the way to go (for now or even >> permanently)? > > If vmx_load_pdptrs only requires P2M_ALLOC then this is already > covered by the call to get_gfn performed in hap_update_paging_modes, > so I don't think there's much point in merging with v1, as forcing > hap_update_paging_modes to unshare the entry won't affect > vmx_load_pdptrs anymore. Oh, I simply mis-remembered what v1 did; for some reason I thought it added a call rather than modifying the query type from alloc to unshare. Jan