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=-4.1 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 E5F6AC433E2 for ; Wed, 2 Sep 2020 16:55:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C0EF820767 for ; Wed, 2 Sep 2020 16:55:07 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=alien8.de header.i=@alien8.de header.b="nk1iqBJ/" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728110AbgIBQzG (ORCPT ); Wed, 2 Sep 2020 12:55:06 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34946 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726742AbgIBQzF (ORCPT ); Wed, 2 Sep 2020 12:55:05 -0400 Received: from mail.skyhub.de (mail.skyhub.de [IPv6:2a01:4f8:190:11c2::b:1457]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0B6A1C061244 for ; Wed, 2 Sep 2020 09:55:04 -0700 (PDT) Received: from zn.tnic (p200300ec2f0d7a00acdede37bac547d6.dip0.t-ipconnect.de [IPv6:2003:ec:2f0d:7a00:acde:de37:bac5:47d6]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.skyhub.de (SuperMail on ZX Spectrum 128k) with ESMTPSA id 923721EC0493; Wed, 2 Sep 2020 18:54:59 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=alien8.de; s=dkim; t=1599065699; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:in-reply-to:in-reply-to: references:references; bh=PVvZberZXPtknBWe89EYmhC7Z/aQIBONE/vTyia2nyg=; b=nk1iqBJ/q9QQ51noWdo/XTN/vMlH1uAzMFA+037bfhPmpqOrgKlm8N93AE0SXxao5Eq+K0 8muK8rVr+FdHAOKGSk+YzsrYnO66B1HB9dzX3zjad67Pc1gEmKpqFgRj90kFX6rwrKY5hU 3gV8M42oy+RnLLVB1kA8bvrvFps7Ov4= Date: Wed, 2 Sep 2020 18:55:01 +0200 From: Borislav Petkov To: peterz@infradead.org Cc: Feng Tang , Thomas Gleixner , Ingo Molnar , "H . Peter Anvin" , Dave Hansen , x86@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH] tools/x86: add kcpuid tool to show raw CPU features Message-ID: <20200902165501.GC21537@zn.tnic> References: <1598514543-90152-1-git-send-email-feng.tang@intel.com> <20200902164538.GN1362448@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200902164538.GN1362448@hirez.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 02, 2020 at 06:45:38PM +0200, peterz@infradead.org wrote: > We really should clear the CPUID bits when the kernel explicitly > disables things. Actually, you want to *disable* the functionality behind it by clearing a bit in CR4 - and yes, not all features have CR4 bits - so that luserspace doesn't "probe" the existence of certain instructions. Example: you can still try to run RDRAND and succeed even if the corresponding CPUID bit is clear. -- Regards/Gruss, Boris. https://people.kernel.org/tglx/notes-about-netiquette