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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id C06A5C433F5 for ; Wed, 4 May 2022 12:43:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1349890AbiEDMrP (ORCPT ); Wed, 4 May 2022 08:47:15 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33102 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235027AbiEDMrH (ORCPT ); Wed, 4 May 2022 08:47:07 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 8D62A33A1B for ; Wed, 4 May 2022 05:43:30 -0700 (PDT) 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 5F0661042; Wed, 4 May 2022 05:43:30 -0700 (PDT) Received: from lakrids (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id E24503FA50; Wed, 4 May 2022 05:43:28 -0700 (PDT) Date: Wed, 4 May 2022 13:43:26 +0100 From: Mark Rutland To: Steven Rostedt Cc: Wang ShaoBo , cj.chengjian@huawei.com, huawei.libin@huawei.com, xiexiuqi@huawei.com, liwei391@huawei.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com, will@kernel.org, zengshun.wu@outlook.com Subject: Re: [RFC PATCH -next v2 3/4] arm64/ftrace: support dynamically allocated trampolines Message-ID: References: <20220316100132.244849-1-bobo.shaobowang@huawei.com> <20220316100132.244849-4-bobo.shaobowang@huawei.com> <20220421100639.03c0d123@gandalf.local.home> <20220421114201.21228eeb@gandalf.local.home> <20220421130648.56b21951@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220421130648.56b21951@gandalf.local.home> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 21, 2022 at 01:06:48PM -0400, Steven Rostedt wrote: > On Thu, 21 Apr 2022 17:27:40 +0100 > Mark Rutland wrote: > > > We can initialize the ops pointer to a default ops that does the whole > > __do_for_each_ftrace_ops() dance. > > OK, I think I understand now. What you are doing is instead of creating a > trampoline that has all the information in the trampoline, you add nops to > all the functions where you can place the information in the nops (before > the function), and then have the trampoline just read that information to > find the ops pointer as well as the function to call. FWIW, I had a go at mocking that up: https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/ftrace/per-callsite-ops Aside from some bodges required to ensure the patch site is suitably aligned (which I think can be cleaned up somewhat), I don't think it looks that bad. I wasn't sure how exactly to wire that up in the core code, so all the patch sites are initialized with a default ops that calls arch_ftrace_ops_list_func(), but it looks like it should be possible to wire that up in the core with some refactoring. > I guess you could have two trampolines as well. One that always calls the > list loop, and one that calls the data stored in front of the function that > was just called the trampoline. As it is always safe to call the loop > function, you could have the call call that trampoline first, set up the > specific data before the function, then call the trampoline that will read > it. I was thinking we could just patch the ops with a default ops that called the list loop, as my patches default them to. > And same thing for tear down. I wasn't sure how teardown was meant to work in general. When we want to remove an ops structure, or a trampoline, how do we ensure those are no longer in use before we remove them? I can see how we can synchronize the updates to the kernel text, but I couldn't spot how we handle a thread being in the middle of a trampoline. Thanks, Mark.