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=-8.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 E3523C282DD for ; Thu, 9 Jan 2020 15:26:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B0D9120673 for ; Thu, 9 Jan 2020 15:26:34 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="ccSAmgpR" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730358AbgAIP0e (ORCPT ); Thu, 9 Jan 2020 10:26:34 -0500 Received: from mail-qv1-f65.google.com ([209.85.219.65]:40520 "EHLO mail-qv1-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728257AbgAIP0d (ORCPT ); Thu, 9 Jan 2020 10:26:33 -0500 Received: by mail-qv1-f65.google.com with SMTP id dp13so3096852qvb.7; Thu, 09 Jan 2020 07:26:33 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=sender:from:date:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=DZMO869N9atXGGz83vPVHWIh7S4cACUuHiNhHyOZd68=; b=ccSAmgpRYS3JuU+Saqy5TN2ryGO5qQ2sWpu4ahtfSrJ2R/vsFZNZzgwx7xKwejvUmd i552h8XAx9Fjqhm9YR1o/jL5XERp4/1cGhkas4dVz/maI3blz44/kx8UQK9kuAnxw4F+ l00lQfaKest6JttZkBORklRnifkGQAtE+SkoFLxzw9XWvVl7xwCmVqZp+pPx3n+9acZR 5rOB88870Dh3CiV4GEFmrPWG80pO7vAlDRkek61EwQjPpUVFx5lWxlkvjN0tBBhl1+di E5rH1CZVGNOjykxnvE6vJXRcjuP6kd5z/xzSGVm788NO1hKkzhTV4SNvt7M08lInDniV U7ew== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:from:date:to:cc:subject:message-id :references:mime-version:content-disposition:in-reply-to:user-agent; bh=DZMO869N9atXGGz83vPVHWIh7S4cACUuHiNhHyOZd68=; b=n1n3Lxz3ePC2MmXZJoT7qoBJmWS1JUuj8xohi2vrkxTdNNHgLOzePYj2K8SDwcgvZi lm7QwYE8/dhjGyzovcnsm6d9/4uSMFhTjplrT7xeVcU/j6dK5GXk5E7Ed06T2sZQnOhq HzREhHaDuGSBduD5q1cv/RyNKQEf4Np29A5vw9Rfb8u/mgUxcoZrnE12cPiN6F4+U2F5 kxgbW1eD0ZG7MVnHDmyj1/RbRnVJOLYhguFInFQgYV1yZARAbWdl5pdkgt06ioGdobmV lEVrykIeaGL+EyKb57GSNNY5XRWBsp5Xbs/Nk/uH2OcTZqaDlHiA3icbbLmr2MaZXBfc C7aA== X-Gm-Message-State: APjAAAWK9qtI7S8P2AfcUgFcY2dAT2ZqD3Hs+7GO/GpmZze6AXKIndu0 OPAWatWn2zn2NrBbTp86V/w= X-Google-Smtp-Source: APXvYqx3RuvAT9wjM40JZ1XavIkf8uBi4ENIt2ba5QEc7qaayoAA/hZW6F5XZUnmejjX+W4JxBdVYQ== X-Received: by 2002:ad4:4354:: with SMTP id q20mr9398920qvs.103.1578583592611; Thu, 09 Jan 2020 07:26:32 -0800 (PST) Received: from rani.riverdale.lan ([2001:470:1f07:5f3::b55f]) by smtp.gmail.com with ESMTPSA id d26sm3159020qka.28.2020.01.09.07.26.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 09 Jan 2020 07:26:32 -0800 (PST) From: Arvind Sankar X-Google-Original-From: Arvind Sankar Date: Thu, 9 Jan 2020 10:26:30 -0500 To: David Laight Cc: 'Sean Christopherson' , Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , "kvm@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH] KVM: x86/mmu: Fix a benign Bitwise vs. Logical OR mixup Message-ID: <20200109152629.GA25610@rani.riverdale.lan> References: <20200108001859.25254-1-sean.j.christopherson@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 09, 2020 at 02:13:48PM +0000, David Laight wrote: > From: Sean Christopherson > > Sent: 08 January 2020 00:19 > > > > Use a Logical OR in __is_rsvd_bits_set() to combine the two reserved bit > > checks, which are obviously intended to be logical statements. Switching > > to a Logical OR is functionally a nop, but allows the compiler to better > > optimize the checks. > > > > Signed-off-by: Sean Christopherson > > --- > > arch/x86/kvm/mmu/mmu.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > > index 7269130ea5e2..72e845709027 100644 > > --- a/arch/x86/kvm/mmu/mmu.c > > +++ b/arch/x86/kvm/mmu/mmu.c > > @@ -3970,7 +3970,7 @@ __is_rsvd_bits_set(struct rsvd_bits_validate *rsvd_check, u64 pte, int level) > > { > > int bit7 = (pte >> 7) & 1, low6 = pte & 0x3f; > > > > - return (pte & rsvd_check->rsvd_bits_mask[bit7][level-1]) | > > + return (pte & rsvd_check->rsvd_bits_mask[bit7][level-1]) || > > ((rsvd_check->bad_mt_xwr & (1ull << low6)) != 0); > > Are you sure this isn't deliberate? > The best code almost certainly comes from also removing the '!= 0'. > You also don't want to convert the expression result to zero. The function is static inline bool, so it's almost certainly a mistake originally. The != 0 is superfluous, but this will get inlined anyway. > > So: > return (pte & rsvd_check->rsvd_bits_mask[bit7][level-1]) | (rsvd_check->bad_mt_xwr & (1ull << low6)); > The code then doesn't have any branches to get mispredicted. > > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales) >