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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_MUTT 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 564A9C31E5D for ; Tue, 18 Jun 2019 22:48:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 37A7220863 for ; Tue, 18 Jun 2019 22:48:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730985AbfFRWsh (ORCPT ); Tue, 18 Jun 2019 18:48:37 -0400 Received: from mga06.intel.com ([134.134.136.31]:7452 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730341AbfFRWsh (ORCPT ); Tue, 18 Jun 2019 18:48:37 -0400 X-Amp-Result: UNSCANNABLE X-Amp-File-Uploaded: False Received: from fmsmga008.fm.intel.com ([10.253.24.58]) by orsmga104.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 18 Jun 2019 15:48:36 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.63,390,1557212400"; d="scan'208";a="160196298" Received: from ranerica-svr.sc.intel.com ([172.25.110.23]) by fmsmga008.fm.intel.com with ESMTP; 18 Jun 2019 15:48:36 -0700 Date: Tue, 18 Jun 2019 15:48:14 -0700 From: Ricardo Neri To: Thomas Gleixner Cc: Ingo Molnar , Borislav Petkov , Ashok Raj , Joerg Roedel , Andi Kleen , Peter Zijlstra , Suravee Suthikulpanit , Stephane Eranian , "Ravi V. Shankar" , Randy Dunlap , x86@kernel.org, linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org, Ricardo Neri , "H. Peter Anvin" , Tony Luck , Clemens Ladisch , Arnd Bergmann , Philippe Ombredanne , Kate Stewart , "Rafael J. Wysocki" Subject: Re: [RFC PATCH v4 05/21] x86/hpet: Reserve timer for the HPET hardlockup detector Message-ID: <20190618224814.GE30488@ranerica-svr.sc.intel.com> References: <1558660583-28561-1-git-send-email-ricardo.neri-calderon@linux.intel.com> <1558660583-28561-6-git-send-email-ricardo.neri-calderon@linux.intel.com> <20190614011454.GA6347@ranerica-svr.sc.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jun 14, 2019 at 06:10:18PM +0200, Thomas Gleixner wrote: > On Thu, 13 Jun 2019, Ricardo Neri wrote: > > > On Tue, Jun 11, 2019 at 09:54:25PM +0200, Thomas Gleixner wrote: > > > On Thu, 23 May 2019, Ricardo Neri wrote: > > > > > > > HPET timer 2 will be used to drive the HPET-based hardlockup detector. > > > > Reserve such timer to ensure it cannot be used by user space programs or > > > > for clock events. > > > > > > > > When looking for MSI-capable timers for clock events, skip timer 2 if > > > > the HPET hardlockup detector is selected. > > > > > > Why? Both the changelog and the code change lack an explanation why this > > > timer is actually touched after it got reserved for the platform. The > > > reservation should make it inaccessible for other things. > > > > hpet_reserve_platform_timers() will give the HPET char driver a data > > structure which specifies which drivers are reserved. In this manner, > > they cannot be used by applications via file opens. The timer used by > > the hardlockup detector should be marked as reserved. > > > > Also, hpet_msi_capability_lookup() populates another data structure > > which is used when obtaining an unused timer for a HPET clock event. > > The timer used by the hardlockup detector should not be included in such > > data structure. > > > > Is this the explanation you would like to see? If yes, I will include it > > in the changelog. > > Yes, the explanation makes sense. The code still sucks. Not really your > fault, but this is not making it any better. > > What bothers me most is the fact that CONFIG_X86_HARDLOCKUP_DETECTOR_HPET > removes one HPET timer unconditionally. It neither checks whether the hpet > watchdog is actually enabled on the command line, nor does it validate > upfront whether the HPET supports FSB delivery. > > That wastes an HPET timer unconditionally for no value. Not that I > personally care much about /dev/hpet, but some older laptops depend on HPET > per cpu timers as the local APIC timer stops in C2/3. So this unconditional > reservation will cause regressions for no reason. > > The proper approach here is to: > > 1) Evaluate the command line _before_ hpet_enable() is invoked > > 2) Check the availability of FSB delivery in hpet_enable() > > Reserve an HPET channel for the watchdog only when #1 and #2 are true. Sure. I will add the explanation in the message commit and only reserve the timer if both of the conditions above are met. Thanks and BR, Ricardo