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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 D94EDCA9EB5 for ; Mon, 4 Nov 2019 13:37:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AA86220B7C for ; Mon, 4 Nov 2019 13:37:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727499AbfKDNhD (ORCPT ); Mon, 4 Nov 2019 08:37:03 -0500 Received: from foss.arm.com ([217.140.110.172]:43160 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727236AbfKDNhD (ORCPT ); Mon, 4 Nov 2019 08:37:03 -0500 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 6117A1FB; Mon, 4 Nov 2019 05:37:02 -0800 (PST) Received: from lakrids.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 2702E3F6C4; Mon, 4 Nov 2019 05:37:00 -0800 (PST) Date: Mon, 4 Nov 2019 13:36:58 +0000 From: Mark Rutland To: Amit Daniel Kachhap Cc: linux-arm-kernel@lists.infradead.org, Steven Rostedt , linux-kernel@vger.kernel.org, catalin.marinas@arm.com, deller@gmx.de, duwe@suse.de, James.Bottomley@HansenPartnership.com, james.morse@arm.com, jeyu@kernel.org, jpoimboe@redhat.com, jthierry@redhat.com, linux-parisc@vger.kernel.org, mingo@redhat.com, peterz@infradead.org, svens@stackframe.org, takahiro.akashi@linaro.org, will@kernel.org Subject: Re: [PATCHv2 1/8] ftrace: add ftrace_init_nop() Message-ID: <20191104133657.GE45140@lakrids.cambridge.arm.com> References: <20191029165832.33606-1-mark.rutland@arm.com> <20191029165832.33606-2-mark.rutland@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.11.1+11 (2f07cb52) (2018-12-01) Sender: linux-parisc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-parisc@vger.kernel.org On Sat, Nov 02, 2019 at 05:49:00PM +0530, Amit Daniel Kachhap wrote: > Hi Mark, Hi Amit, Thanks for the review! > On 10/29/19 10:28 PM, Mark Rutland wrote: > > Architectures may need to perform special initialization of ftrace > > callsites, and today they do so by special-casing ftrace_make_nop() when > > the expected branch address is MCOUNT_ADDR. In some cases (e.g. for > > patchable-function-entry), we don't have an mcount-like symbol and don't > s/an mcount/a mcount. > > want a synthetic MCOUNT_ADDR, but we may need to perform some > > initialization of callsites. > > > > To make it possible to separate initialization from runtime > > modification, and to handle cases without an mcount-like symbol, this > Same as above. Using 'an' in both of these cases is correct, even though 'mcount' starts with a consonant, since it's pronounced as-if it were 'emcount'. I will leave this as-is. > > patch adds an optional ftrace_init_nop() function that architectures can > > implement, which does not pass a branch address. > > > > Where an architecture does not provide ftrace_init_nop(), we will fall > > back to the existing behaviour of calling ftrace_make_nop() with > > MCOUNT_ADDR. > > > > At the same time, ftrace_code_disable() is renamed to > > ftrace_nop_initialize() to make it clearer that it is intended to > > intialize a callsite into a disabled state, and is not for disabling a > > callsite that has been runtime enabled. The kerneldoc description of rec > > arguments is updated to cover non-mcount callsites. > > > > Signed-off-by: Mark Rutland > > Reviewed-by: Ard Biesheuvel > > Cc: Ingo Molnar > > Cc: Steven Rostedt > > Cc: Torsten Duwe > > --- > > include/linux/ftrace.h | 35 ++++++++++++++++++++++++++++++++--- > > kernel/trace/ftrace.c | 6 +++--- > > 2 files changed, 35 insertions(+), 6 deletions(-) > > > > diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h > > index 8a8cb3c401b2..9867d90d635e 100644 > > --- a/include/linux/ftrace.h > > +++ b/include/linux/ftrace.h > > @@ -499,7 +499,7 @@ static inline int ftrace_disable_ftrace_graph_caller(void) { return 0; } > > /** > > * ftrace_make_nop - convert code into nop > > * @mod: module structure if called by module load initialization > > - * @rec: the mcount call site record > > + * @rec: the call site record (e.g. mcount/fentry) > > * @addr: the address that the call site should be calling > > * > > * This is a very sensitive operation and great care needs > > @@ -520,9 +520,38 @@ static inline int ftrace_disable_ftrace_graph_caller(void) { return 0; } > > extern int ftrace_make_nop(struct module *mod, > > struct dyn_ftrace *rec, unsigned long addr); > > + > > +/** > > + * ftrace_init_nop - initialize a nop call site > > + * @mod: module structure if called by module load initialization > > + * @rec: the call site record (e.g. mcount/fentry) > > + * > > + * This is a very sensitive operation and great care needs > > + * to be taken by the arch. The operation should carefully > > + * read the location, check to see if what is read is indeed > > + * what we expect it to be, and then on success of the compare, > > + * it should write to the location. > > + * > > + * The code segment at @rec->ip should contain the contents created by > > + * the compiler > Nit: Will it be better to write it as "@rec->ip should store the adjusted > ftrace entry address of the call site" or something like that. This was the specific wording requested by Steve, and it's trying to describe the instructions at rec->ip, rather than the value of rec->ip, so I think it's better to leave this as-is. > > + * > > + * Return must be: > > + * 0 on success > > + * -EFAULT on error reading the location > > + * -EINVAL on a failed compare of the contents > > + * -EPERM on error writing to the location > > + * Any other value will be considered a failure. > > + */ > > +#ifndef ftrace_init_nop > > +static inline int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec) > > +{ > > + return ftrace_make_nop(mod, rec, MCOUNT_ADDR); > > +} > > +#endif > > + > Now that ftrace_init_nop is also an arch implemented function so it may be > added in Documentation/trace/ftrace-design.rst along with ftrace_make_nop. > In general also, adding some description about patchable-function-entry > in kernel Documentation will be useful. I agree that would be a good thing, but as Steve suggests I will leave this for subsequent rework, as I think that also implies more rework/renaming in the core code (e.g. to more cleanly separate the notion of a callsite from mcount specifically). Steve, if you'd like help with that (or review), I'd be happy to help. Thanks, Mark.