From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759932AbcIMSHg (ORCPT ); Tue, 13 Sep 2016 14:07:36 -0400 Received: from mx1.redhat.com ([209.132.183.28]:39534 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759627AbcIMSHd (ORCPT ); Tue, 13 Sep 2016 14:07:33 -0400 Subject: Re: [PATCH] x86/apic: Use byte array apic_version[], not int array. Saves up to 96k To: Thomas Gleixner , Borislav Petkov References: <20160909083204.781-1-dvlasenk@redhat.com> <20160911093149.foleg464zmcwf6w5@pd.tnic> Cc: Ingo Molnar , Andy Lutomirski , "H. Peter Anvin" , Brian Gerst , x86@kernel.org, linux-kernel@vger.kernel.org, Mike Travis From: Denys Vlasenko Message-ID: Date: Tue, 13 Sep 2016 20:07:29 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Tue, 13 Sep 2016 18:07:32 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/13/2016 05:33 PM, Thomas Gleixner wrote: > On Sun, 11 Sep 2016, Borislav Petkov wrote: >> On Fri, Sep 09, 2016 at 10:32:04AM +0200, Denys Vlasenko wrote: >>> This array is [MAX_LOCAL_APIC], and MAX_LOCAL_APIC can easily be up to 32k. >>> >>> This patch changes apic_version[] array elements from int to u8 - >>> APIC version values as of year 2016 are no larger than 0x1f on all known CPUs. >>> Version field in the APIC register is 8 bit wide - not likely >>> to overflow byte range in foreseeable future. >>> >>> The "ver" argument of generic_processor_info(id,ver), which goes into apic_version[id], >>> is also changed from int to u8: make it obvious that assignment can't overflow. >>> >>> generic_processor_info() has four callsites, none of them can put an out-of-range value >>> into this argument. >> >> Right, so I dug a bit into this and found: >> >> http://marc.info/?l=linux-kernel&m=123230551709711 >> >> and >> >> b2b815d80a5c ("x86: put trigger in to detect mismatched apic versions") >> >> It is from 2009 and I don't know how relevant 16-bit APIC IDs are >> anymore... I guess you probably want to run this by SGI folk first. >> >> Otherwise I was going to propose to kill that apic_version array >> altogether and cache only the version of the previous CPU and compare it >> to the current one to catch mismatches... > > Yeah, the idea was back then to eliminate the array, but we wanted to make > sure that we don't have systems out in the wild which have different apic > versions. I really doubt that we can deal with that proper, so having a > single version entry and yelling loudly when we detect a mismatch is good > enough. Makes sense. I'll send a patch