From patchwork Wed Jan 6 14:14:18 2016 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Stefano Stabellini X-Patchwork-Id: 7968461 Return-Path: X-Original-To: patchwork-xen-devel@patchwork.kernel.org Delivered-To: patchwork-parsemail@patchwork1.web.kernel.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.136]) by patchwork1.web.kernel.org (Postfix) with ESMTP id 0347F9F1C0 for ; Wed, 6 Jan 2016 14:17:17 +0000 (UTC) Received: from mail.kernel.org (localhost [127.0.0.1]) by mail.kernel.org (Postfix) with ESMTP id 494662012B for ; Wed, 6 Jan 2016 14:17:16 +0000 (UTC) Received: from lists.xen.org (lists.xenproject.org [50.57.142.19]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 73F152011D for ; Wed, 6 Jan 2016 14:17:15 +0000 (UTC) Received: from localhost ([127.0.0.1] helo=lists.xen.org) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1aGorP-0005Rv-Mj; Wed, 06 Jan 2016 14:14:47 +0000 Received: from mail6.bemta14.messagelabs.com ([193.109.254.103]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1aGorN-0005Rf-EB for xen-devel@lists.xensource.com; Wed, 06 Jan 2016 14:14:45 +0000 Received: from [193.109.254.147] by server-7.bemta-14.messagelabs.com id 31/9E-28221-4512D865; Wed, 06 Jan 2016 14:14:44 +0000 X-Env-Sender: prvs=8066dafa4=Stefano.Stabellini@citrix.com X-Msg-Ref: server-15.tower-27.messagelabs.com!1452089682!15074217!1 X-Originating-IP: [66.165.176.63] X-SpamReason: No, hits=0.0 required=7.0 tests=sa_preprocessor: VHJ1c3RlZCBJUDogNjYuMTY1LjE3Ni42MyA9PiAzMDYwNDg=\n, received_headers: No Received headers X-StarScan-Received: X-StarScan-Version: 7.35.1; banners=-,-,- X-VirusChecked: Checked Received: (qmail 25285 invoked from network); 6 Jan 2016 14:14:43 -0000 Received: from smtp02.citrix.com (HELO SMTP02.CITRIX.COM) (66.165.176.63) by server-15.tower-27.messagelabs.com with RC4-SHA encrypted SMTP; 6 Jan 2016 14:14:43 -0000 X-IronPort-AV: E=Sophos;i="5.20,529,1444694400"; d="scan'208";a="329336800" Date: Wed, 6 Jan 2016 14:14:18 +0000 From: Stefano Stabellini X-X-Sender: sstabellini@kaball.uk.xensource.com To: David Vrabel In-Reply-To: <568D071A.7050009@citrix.com> Message-ID: References: <568D071A.7050009@citrix.com> User-Agent: Alpine 2.02 (DEB 1266 2009-07-14) MIME-Version: 1.0 X-DLP: MIA2 Cc: xen-devel@lists.xensource.com, liuling-it@360.cn, qemu-devel@nongnu.org, Stefano Stabellini Subject: Re: [Xen-devel] [PATCH] xenfb.c: avoid expensive loops when prod <= out_cons X-BeenThere: xen-devel@lists.xen.org X-Mailman-Version: 2.1.13 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org X-Spam-Status: No, score=-4.2 required=5.0 tests=BAYES_00, RCVD_IN_DNSWL_MED, UNPARSEABLE_RELAY autolearn=unavailable version=3.3.1 X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on mail.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP On Wed, 6 Jan 2016, David Vrabel wrote: > On 06/01/16 12:08, Stefano Stabellini wrote: > > If the frontend sets out_cons to a value higher than out_prod, it will > > cause xenfb_handle_events to loop about 2^32 times. Avoid that by using > > better checks at the beginning of the function. > > You can't use less than to compare prod and cons because they wrap. > > You need to compare (prod - cons) against ring size (or similar) to > check for overflow. See RING_REQUEST_PROD_OVERFLOW() etc. Yes, you are right. I think that the right fix should be: diff --git a/hw/display/xenfb.c b/hw/display/xenfb.c index 4e2a27a..594baff 100644 --- a/hw/display/xenfb.c +++ b/hw/display/xenfb.c @@ -789,8 +789,9 @@ static void xenfb_handle_events(struct XenFB *xenfb) prod = page->out_prod; out_cons = page->out_cons; - if (prod == out_cons) - return; + if (prod - out_cons >= XENFB_OUT_RING_LEL) { + return; + } xen_rmb(); /* ensure we see ring contents up to prod */ for (cons = out_cons; cons != prod; cons++) { union xenfb_out_event *event = &XENFB_OUT_RING_REF(page, cons);