Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752298AbbF2TtO (ORCPT ); Mon, 29 Jun 2015 15:49:14 -0400 Received: from terminus.zytor.com ([198.137.202.10]:33918 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752182AbbF2TtH (ORCPT ); Mon, 29 Jun 2015 15:49:07 -0400 Message-ID: <5591A103.6020104@zytor.com> Date: Mon, 29 Jun 2015 12:48:19 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: Ingo Molnar , Borislav Petkov CC: Mike Galbraith , Ingo Molnar , LKML , Thomas Gleixner , Andy Lutomirski , Denys Vlasenko , Oleg Nesterov , Dave Hansen Subject: Re: [all better] Re: regression: massive trouble with fpu rework References: <1435386316.3664.23.camel@gmail.com> <1435393129.3490.7.camel@gmail.com> <20150627082514.GA10894@gmail.com> <1435395328.6545.10.camel@gmail.com> <20150629064008.GA16251@gmail.com> <1435566329.2900.1.camel@gmail.com> <20150629083302.GA13113@pd.tnic> <20150629093504.GA20600@gmail.com> In-Reply-To: <20150629093504.GA20600@gmail.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1120 Lines: 34 On 06/29/2015 02:35 AM, Ingo Molnar wrote: > > Indeed, I bet that makes a difference! > > I wish that 'unmasking' logic came with more comments: > > - Why do BIOSen ever mask CPUIDs? > To work around bugs in legacy operating systems. > - Why do we unmask the masking? Because we don't have those specific bugs. > - Why doesn't the kernel keep on working just fine even if certain CPUID aspects > are turned off? Because it exercises code paths that are otherwise impossible, for example, it exposes the XSAVE capability without exposing the XSAVE information in higher CPUID leaves. The other option would be to have a list of CPU features that should be turned off whenever the CPUID leaf maximum is too low, but it gives a better user experience to just override the BIOS capping and then we have fewer code paths in the kernel to worry about. -hpa -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/