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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 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 828DBC433DF for ; Thu, 21 May 2020 03:16:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 58D97206C3 for ; Thu, 21 May 2020 03:16:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728110AbgEUDQR (ORCPT ); Wed, 20 May 2020 23:16:17 -0400 Received: from foss.arm.com ([217.140.110.172]:39448 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727825AbgEUDQR (ORCPT ); Wed, 20 May 2020 23:16:17 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 79F5530E; Wed, 20 May 2020 20:16:16 -0700 (PDT) Received: from [10.163.75.69] (unknown [10.163.75.69]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7F9A23F52E; Wed, 20 May 2020 20:16:14 -0700 (PDT) From: Anshuman Khandual Subject: Re: [PATCH] arm64/cpufeature: Move BUG_ON() inside get_arm64_ftr_reg() To: Will Deacon , Catalin Marinas Cc: linux-arm-kernel@lists.infradead.org, mark.rutland@arm.com, Suzuki K Poulose , Mark Brown , linux-kernel@vger.kernel.org References: <1589937774-20479-1-git-send-email-anshuman.khandual@arm.com> <20200520122012.GA25815@willie-the-truck> <20200520154711.GD18302@gaia> <20200520173953.GA27629@willie-the-truck> Message-ID: Date: Thu, 21 May 2020 08:45:38 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20200520173953.GA27629@willie-the-truck> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/20/2020 11:09 PM, Will Deacon wrote: > On Wed, May 20, 2020 at 04:47:11PM +0100, Catalin Marinas wrote: >> On Wed, May 20, 2020 at 01:20:13PM +0100, Will Deacon wrote: >>> On Wed, May 20, 2020 at 06:52:54AM +0530, Anshuman Khandual wrote: >>>> There is no way to proceed when requested register could not be searched in >>>> arm64_ftr_reg[]. Requesting for a non present register would be an error as >>>> well. Hence lets just BUG_ON() when the search fails in get_arm64_ftr_reg() >>>> rather than checking for return value and doing the same in some individual >>>> callers. >>>> >>>> But there are some callers that dont BUG_ON() upon search failure. It adds >>>> an argument 'failsafe' that provides required switch between callers based >>>> on whether they could proceed or not. >>>> >>>> Cc: Catalin Marinas >>>> Cc: Will Deacon >>>> Cc: Suzuki K Poulose >>>> Cc: Mark Brown >>>> Cc: linux-arm-kernel@lists.infradead.org >>>> Cc: linux-kernel@vger.kernel.org >>>> >>>> Signed-off-by: Anshuman Khandual >>>> --- >>>> Applies on next-20200518 that has recent cpufeature changes from Will. >>>> >>>> arch/arm64/kernel/cpufeature.c | 26 +++++++++++++------------- >>>> 1 file changed, 13 insertions(+), 13 deletions(-) >>>> >>>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c >>>> index bc5048f152c1..62767cc540c3 100644 >>>> --- a/arch/arm64/kernel/cpufeature.c >>>> +++ b/arch/arm64/kernel/cpufeature.c >>>> @@ -557,7 +557,7 @@ static int search_cmp_ftr_reg(const void *id, const void *regp) >>>> * - NULL on failure. It is upto the caller to decide >>>> * the impact of a failure. >>>> */ >>>> -static struct arm64_ftr_reg *get_arm64_ftr_reg(u32 sys_id) >>>> +static struct arm64_ftr_reg *get_arm64_ftr_reg(u32 sys_id, bool failsafe) >>> >>> Generally, I'm not a big fan of boolean arguments because they are really >>> opaque at the callsite. It also seems bogus to me that we don't trust the >>> caller to pass a valid sys_id, but we trust it to get "failsafe" right, >>> which seems to mean "I promise to check the result isn't NULL before >>> dereferencing it." >>> >>> So I don't see how this patch improves anything. I'd actually be more >>> inclined to stick a WARN() in get_arm64_ftr_reg() when it returns NULL and >>> have the callers handle NULL by returning early, getting rid of all the >>> BUG_ONs in here. Sure, the system might end up in a funny state, but we >>> WARN()d about it and tried to keep going (and Linus has some strong opinions >>> on this too). >> >> Such WARN can be triggered by the user via emulate_sys_reg(), so we >> can't really have it in get_arm64_ftr_reg() without a 'failsafe' option. > > Ah yes, that would be bad. In which case, I don't think the existing code > should change. The existing code has BUG_ON() in three different callers doing exactly the same thing that can easily be taken care in get_arm64_ftr_reg() itself. As mentioned before an enum variable (as preferred - over a bool) can still preserve the existing behavior for emulate_sys_reg(). IMHO these are very good reasons for us to change the code which will make it cleaner while also removing three redundant BUG_ON() instances. Hence I will request you to please reconsider this proposal. - Anshuman