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=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 4E324C433E0 for ; Sun, 24 May 2020 23:53:12 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id C6F3D2075D for ; Sun, 24 May 2020 23:53:11 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="b7o4eDIp" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C6F3D2075D Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:Date: Message-ID:From:References:To:Subject:Reply-To:Content-ID:Content-Description :Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=+ZhEW/A3BI93MYAZGRE66cEfn2eKX7AsQVPD9ssy5qA=; b=b7o4eDIpstzfv0 XjHc8FmaRxKBuOkcR+VH85ZhEEk5q0BZqNIgvJYA5WV8T/rvV+zcxSRusnyLUF0ktSUoYledp7wMo oCDuLmE5KTeXM75J3uKvU9lWd5+J651ZZqg9zbN9Xm5ZdDzWRPi1yAh+eRw/hsr8m70BHe3WreOnY T5oORGo3Hb/MTWGijyDfxSmFTMDMlcFisU94YswxSBclc7ftq6dwzUo7DWMxnetj7xQLa/XnjuF4i uHC+S1hh905XtaS9BDOmyjnVuXulOFtXKKe9QopamYJYCeHUYqOzIdHKXCDCNiUUHxg9HFoTTIecW uKXiZ75KKjMOKLSCXt8g==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jd0QZ-0000WE-CG; Sun, 24 May 2020 23:53:11 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jd0QW-0000VP-7H for linux-arm-kernel@lists.infradead.org; Sun, 24 May 2020 23:53:09 +0000 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 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-Language: en-US X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200524_165308_348898_DDF21CAC X-CRM114-Status: GOOD ( 20.69 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: mark.rutland@arm.com, Mark Brown , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Suzuki K Poulose Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.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; } _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel