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=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, 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 DA7CCC433DF for ; Sun, 24 May 2020 23:53:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B971D2075D for ; Sun, 24 May 2020 23:53:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388509AbgEXXxG (ORCPT ); Sun, 24 May 2020 19:53:06 -0400 Received: from foss.arm.com ([217.140.110.172]:34494 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388124AbgEXXxF (ORCPT ); Sun, 24 May 2020 19:53:05 -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 D1F6E30E; Sun, 24 May 2020 16:53:04 -0700 (PDT) Received: from [10.163.76.115] (unknown [10.163.76.115]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id C41C63F52E; Sun, 24 May 2020 16:53:02 -0700 (PDT) Subject: Re: [PATCH] arm64/cpufeature: Move BUG_ON() inside get_arm64_ftr_reg() To: Catalin Marinas , Will Deacon 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> <20200521162212.GK6608@willie-the-truck> <20200521165916.GF11507@gaia> From: Anshuman Khandual Message-ID: Date: Mon, 25 May 2020 05:22:23 +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: <20200521165916.GF11507@gaia> 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 05/21/2020 10:29 PM, Catalin Marinas wrote: > On Thu, May 21, 2020 at 05:22:15PM +0100, Will Deacon wrote: >> On Thu, May 21, 2020 at 08:45:38AM +0530, Anshuman Khandual wrote: >>> 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. >> >> Hmm, then how about trying my proposal with the WARN_ON(), but having a >> get_arm64_ftr_reg_nowarn() variant for the user emulation case? > > That works for me, get_arm64_ftr_reg() would be a wrapper over the > _nowarn function with the added WARN_ON. Sure, will do. > > read_sanitised_ftr_reg() would need to return something though. Would > all 0s be ok? I think it works as long as we don't have negative CPUID > fields. Just trying to understand. If get_arm64_ftr_reg() returns NULL, then read_sanitised_ftr_reg() should also return 0 for that non existent register (already been warned in get_arm64_ftr_reg). @@ -961,8 +972,8 @@ u64 read_sanitised_ftr_reg(u32 id) { struct arm64_ftr_reg *regp = get_arm64_ftr_reg(id); - /* We shouldn't get a request for an unsupported register */ - BUG_ON(!regp); + if (!regp) + return 0; return regp->sys_val; }