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=-8.5 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,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 776A0C433E1 for ; Thu, 21 May 2020 16:22:23 +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 4BAA62084C for ; Thu, 21 May 2020 16:22:23 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="nCnMzbBw"; dkim=fail reason="signature verification failed" (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="gTs5Yccr" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4BAA62084C Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org 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:References: Message-ID:Subject:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=F5MAZ3c3axgN1DrtueqKS25wkLVbMSs1WOXj6Xbi68Q=; b=nCnMzbBwc6mjsl eTTtTEBQdhsIY59s+8f/9VlFOSbaDdjEhnngteIhN/Cerq5796my7ELS60/+HmOg9SiYXPs/E3AXg XWWoOMgVAX5KqM1siGHPZoGIGuWX4PunEVaDcWgJndxO/OJ+xvQ6kmqsr+VzMYBa8xAmOPhKXccbj FHzSuXUiIZ+qx4ywZeC7J0KP13SRtBlh9z4XltW4+uxc3UElNcGrYRFeFx11s2YU8AQXBUBw1dD8x bBnWykv57rbJdxKYtPloN/cUvWPx71RVAyPZayRrKd5lYKNyrxdAXB59xQcZvE2EmNypXD3VBginT LR4z6r63bKWLugXFBNeQ==; 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 1jbnxe-0002ZJ-7j; Thu, 21 May 2020 16:22:22 +0000 Received: from mail.kernel.org ([198.145.29.99]) by bombadil.infradead.org with esmtps (Exim 4.92.3 #3 (Red Hat Linux)) id 1jbnxc-0002Yh-7A for linux-arm-kernel@lists.infradead.org; Thu, 21 May 2020 16:22:21 +0000 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 7586E20748; Thu, 21 May 2020 16:22:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1590078139; bh=5WuN5J6JrrL0Ru1Jeq5Ugd2Xq4HNCOyuxSZAqZ9/hJ4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=gTs5YccrNnx7oMz2b9CUi2XjtNt1vO4YfZvow8YVbqCY8nR5T0dOo2tDlQ2ApwHTJ ICYraVFGWQO4IbdRvn9DPI5exaKFpwsq2vu9Fjw46Rwd0vatycQJgF1ofc3kpom1D7 j4vqjgcvEJD3qNHpgWksTasFY55N2TOrPt+Xccnc= Date: Thu, 21 May 2020 17:22:15 +0100 From: Will Deacon To: Anshuman Khandual Subject: Re: [PATCH] arm64/cpufeature: Move BUG_ON() inside get_arm64_ftr_reg() Message-ID: <20200521162212.GK6608@willie-the-truck> References: <1589937774-20479-1-git-send-email-anshuman.khandual@arm.com> <20200520122012.GA25815@willie-the-truck> <20200520154711.GD18302@gaia> <20200520173953.GA27629@willie-the-truck> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200521_092220_295689_7A201C88 X-CRM114-Status: GOOD ( 22.78 ) 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, Suzuki K Poulose , Catalin Marinas , linux-kernel@vger.kernel.org, Mark Brown , linux-arm-kernel@lists.infradead.org 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 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? Will _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel