From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752438AbdEPOAQ (ORCPT ); Tue, 16 May 2017 10:00:16 -0400 Received: from terminus.zytor.com ([65.50.211.136]:41451 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751498AbdEPOAO (ORCPT ); Tue, 16 May 2017 10:00:14 -0400 Subject: Re: [PATCH] X86: don't report PAT on CPUs that don't support it To: Mikulas Patocka , Thomas Gleixner , Ingo Molnar Cc: x86@kernel.org, linux-kernel@vger.kernel.org References: From: "H. Peter Anvin" Message-ID: <235765f2-77dd-6210-17a1-ca67eecc5983@zytor.com> Date: Tue, 16 May 2017 06:57:07 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.1.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/18/17 12:07, Mikulas Patocka wrote: > > However, on AMD K6-3 CPU, the processor initialization code never calls > pat_init() and so __pat_enabled stays 1 and the function pat_enabled() > returns true, even though the K6-3 CPU doesn't support PAT. > OK, now I'm wondering: are you actually *using* said K6-3 machine, and if so, are you actually dependent on write combining on it? The reason I'm asking is because I would personally like to completely remove the support for using MTRRs to create WC mappings, as it only affects a handful of ancient CPUs: Pentium Pro, Pentium II, K6-*, and possibly some Cyrix/Centaur part. Earlier CPUs didn't have WC, but could set WB, WT or UC via the page tables without needing the PAT MSR, and newer CPUs have PAT. -hpa