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=-3.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_PASS 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 70533ECE564 for ; Tue, 18 Sep 2018 17:47:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2431D214C2 for ; Tue, 18 Sep 2018 17:47:49 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2431D214C2 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-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730537AbeIRXV1 (ORCPT ); Tue, 18 Sep 2018 19:21:27 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:48754 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729583AbeIRXV1 (ORCPT ); Tue, 18 Sep 2018 19:21:27 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 01220ED1; Tue, 18 Sep 2018 10:47:47 -0700 (PDT) Received: from [10.119.49.13] (unknown [10.119.49.13]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id DB1C93F703; Tue, 18 Sep 2018 10:47:44 -0700 (PDT) Subject: Re: [PATCH v5 03/27] arm64: alternative: Apply alternatives early in boot process To: Daniel Thompson , Julien Thierry Cc: Suzuki K Poulose , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, joel@joelfernandes.org, marc.zyngier@arm.com, mark.rutland@arm.com, christoffer.dall@arm.com, catalin.marinas@arm.com, will.deacon@arm.com References: <1535471497-38854-1-git-send-email-julien.thierry@arm.com> <1535471497-38854-4-git-send-email-julien.thierry@arm.com> <3becf020-b230-beb8-b304-d8097377f234@arm.com> <78781d82-e5c4-c590-6c0c-e7d2db456bf9@arm.com> <20180917234400.ctq3w2u6fi5s5coj@holly.lan> From: James Morse Message-ID: <46463b3d-7ceb-f5a5-4061-e988bfc53a6e@arm.com> Date: Tue, 18 Sep 2018 18:47:43 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 MIME-Version: 1.0 In-Reply-To: <20180917234400.ctq3w2u6fi5s5coj@holly.lan> Content-Type: text/plain; charset=utf-8; format=flowed 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 Hi Daniel, Julien, On 09/18/2018 12:44 AM, Daniel Thompson wrote: > On Wed, Sep 12, 2018 at 05:49:09PM +0100, Julien Thierry wrote: >>>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c >>>> index 3bc1c8b..0d1e41e 100644 >>>> --- a/arch/arm64/kernel/cpufeature.c >>>> +++ b/arch/arm64/kernel/cpufeature.c >>>> @@ -52,6 +52,8 @@ >>>> DECLARE_BITMAP(cpu_hwcaps, ARM64_NCAPS); >>>> EXPORT_SYMBOL(cpu_hwcaps); >>>> +unsigned long boot_capabilities; >>>> + >>>> /* >>>> * Flag to indicate if we have computed the system wide >>>> * capabilities based on the boot time active CPUs. This >>>> @@ -1375,6 +1377,9 @@ static void __update_cpu_capabilities(const struct arm64_cpu_capabilities *caps, >>>> if (!cpus_have_cap(caps->capability) && caps->desc) >>>> pr_info("%s %s\n", info, caps->desc); >>>> cpus_set_cap(caps->capability); >>> >>> Hmm, the bitmap behind cpus_set_cap() is what cpus_have_cap() in >>> __apply_alternatives() looks at. If you had a call to __apply_alternatives after >>> update_cpu_capabilities(SCOPE_BOOT_CPU), but before any others, it would only >>> apply those alternatives... >>> >>> (I don't think there is a problem re-applying the same alternative, but I >>> haven't checked). >> Interesting idea. If someone can confirm that patching alternatives twice is >> safe, I think it would make things simpler. Sounds good, I think we need to avoid adding a limit to the number of caps. The extra-work is inefficient, but if it saves merging those lists as part of this series its probably fine. (we only do this stuff once during boot) > Early versions of this patch applied the alternatives twice. I never > noticed any problems with double patching (second time round it will > write out code that is identical to what is already there so it is > merely inefficient rather than unsafe. For the regular kind, I agree. But we've recently grown some fancy dynamic patching where the code is generated at runtime, instead of swapping in an alternative sequence. Details in commit dea5e2a4 ("arm64: alternatives: Add dynamic patching feature"). Its unlikely we would ever apply these twice as they can't have a scope, ... and they all look safe. Thanks, James