From nobody Sat May 4 12:46:17 2024 Delivered-To: importer@patchew.org Received-SPF: pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) client-ip=208.118.235.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Authentication-Results: mx.zoho.com; spf=pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; Return-Path: Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) by mx.zohomail.com with SMTPS id 1487847653091248.05370650224484; Thu, 23 Feb 2017 03:00:53 -0800 (PST) Received: from localhost ([::1]:57794 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cgr8m-0005Xw-4B for importer@patchew.org; Thu, 23 Feb 2017 06:00:52 -0500 Received: from eggs.gnu.org ([2001:4830:134:3::10]:48361) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cgr7X-0004uH-Ka for qemu-devel@nongnu.org; Thu, 23 Feb 2017 05:59:36 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cgr7U-000855-Jq for qemu-devel@nongnu.org; Thu, 23 Feb 2017 05:59:35 -0500 Received: from mx1.redhat.com ([209.132.183.28]:34322) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cgr7U-00084v-Ep for qemu-devel@nongnu.org; Thu, 23 Feb 2017 05:59:32 -0500 Received: from int-mx09.intmail.prod.int.phx2.redhat.com (int-mx09.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 95D1E804E2; Thu, 23 Feb 2017 10:59:32 +0000 (UTC) Received: from t460.redhat.com (ovpn-117-166.ams2.redhat.com [10.36.117.166]) by int-mx09.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with ESMTP id v1NAxTwf020585; Thu, 23 Feb 2017 05:59:30 -0500 From: "Daniel P. Berrange" To: qemu-devel@nongnu.org Date: Thu, 23 Feb 2017 10:59:22 +0000 Message-Id: <20170223105922.22989-1-berrange@redhat.com> X-Scanned-By: MIMEDefang 2.68 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.27]); Thu, 23 Feb 2017 10:59:32 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.132.183.28 Subject: [Qemu-devel] [PATCH] os: don't corrupt pre-existing memory-backend data with prealloc X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Jitendra Kolhe , Paolo Bonzini , Stefan Hajnoczi , Michal Privoznik Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail: RSF_0 Z_629925259 SPT_0 Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" When using a memory-backend object with prealloc turned on, QEMU will memset() the first byte in every memory page to zero. While this might have been acceptable for memory backends associated with RAM, this corrupts application data for NVDIMMs. Instead of setting every page to zero, read the current byte value and then just write that same value back, so we are not corrupting the original data. Signed-off-by: Daniel P. Berrange --- I'm unclear if this is actually still safe in practice ? Is the compiler permitted to optimize away the read+write since it doesn't change the memory value. I'd hope not, but I've been surprised before... IMHO this is another factor in favour of requesting an API from the kernel to provide the prealloc behaviour we want. util/oslib-posix.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/util/oslib-posix.c b/util/oslib-posix.c index 35012b9..8f5b656 100644 --- a/util/oslib-posix.c +++ b/util/oslib-posix.c @@ -355,7 +355,8 @@ void os_mem_prealloc(int fd, char *area, size_t memory,= Error **errp) =20 /* MAP_POPULATE silently ignores failures */ for (i =3D 0; i < numpages; i++) { - memset(area + (hpagesize * i), 0, 1); + char val =3D *(area + (hpagesize * i)); + memset(area + (hpagesize * i), 0, val); } } =20 --=20 2.9.3