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=-4.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, URIBL_BLOCKED,USER_AGENT_MUTT 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 C1ABDC43387 for ; Fri, 4 Jan 2019 22:42:05 +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 7D6622173B for ; Fri, 4 Jan 2019 22:42:05 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="Dav6chek" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7D6622173B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=lst.de 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=RYR1R0FuafXcLGI9Ki23RkFd3YusiI5DfZ0rkT3tVzg=; b=Dav6chekpEp4Rs I4N0v0PxHOTDLn6IIWGct5jemFKvBDY5l996qKysSW1lE9/FX8MWc6/+zd1kqz3LPx28bGysqHT2D cwhte5GZK+M3sP197Ixc9WmsbyAiFonZjqoyYYe1lTVDS1NRJbsNzwPMSHLY+2xTpGjVEox1tnyhJ 0xdnM3BJrFzLdqS2fa/g4YCzA1Ozvug64Sb4ct80fGyfIrRj08s9Q3Lx1SYyUGFAqtCdogkthTG84 kpMeXV5IUatbQV2hY6aq0bGtbmIwD638cbXumbVS0VUpekh/NCgnUip3MU7t543XW+vpHTTrGGADY f3TNJrPqkW5HSDt0zPZQ==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.90_1 #2 (Red Hat Linux)) id 1gfYAE-0001Fr-Gc; Fri, 04 Jan 2019 22:42:02 +0000 Received: from verein.lst.de ([213.95.11.211] helo=newverein.lst.de) by bombadil.infradead.org with esmtps (Exim 4.90_1 #2 (Red Hat Linux)) id 1gfYAB-0001Ej-2t for linux-arm-kernel@lists.infradead.org; Fri, 04 Jan 2019 22:42:01 +0000 Received: by newverein.lst.de (Postfix, from userid 107) id 0C2A268DD2; Fri, 4 Jan 2019 23:41:57 +0100 (CET) Received: from lst.de (p5B33F989.dip0.t-ipconnect.de [91.51.249.137]) by newverein.lst.de (Postfix) with ESMTPSA id 91DA567358; Fri, 4 Jan 2019 23:41:53 +0100 (CET) Date: Fri, 4 Jan 2019 23:41:45 +0100 From: Torsten Duwe To: Steven Rostedt Subject: Re: [PATCH v6] arm64: implement ftrace with regs Message-ID: <20190104224145.GA28236@lst.de> References: <20190104141053.360F768D93@newverein.lst.de> <20190104175017.GA7157@lakrids.cambridge.arm.com> <20190104130648.02657f3f@gandalf.local.home> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20190104130648.02657f3f@gandalf.local.home> 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-20190104_144159_282642_C0C54E60 X-CRM114-Status: GOOD ( 17.95 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , Arnd Bergmann , Julien Thierry , Catalin Marinas , Ard Biesheuvel , Will Deacon , linux-kernel@vger.kernel.org, AKASHI Takahiro , Ingo Molnar , Josh Poimboeuf , Amit Daniel Kachhap , live-patching@vger.kernel.org, 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 Fri, Jan 04, 2019 at 01:06:48PM -0500, Steven Rostedt wrote: > On Fri, 4 Jan 2019 17:50:18 +0000 > Mark Rutland wrote: > > > At Linux Plumbers, I had a conversation with Steve Rostedt, and we came > > to the conclusion that (withut heavyweight synchronization) patching two > > NOPs at runtime isn't safe, since a CPU might have executed the first > > NOP as a NOP before another CPU patches both instructions. So a CPU > > might execute: > > > > NOP > > BL ftrace_regs_caller > > > > ... rather than the expected: > > > > MOV X9, X30 > > BL ftrace_regs_caller > > > > ... and therefore X9 contains some UNKNOWN value, rather than the > > original LR value. I'm perfectly aware of that; an earlier version had barriers, attempting to avoid just that, which Mark(?) wrote weren't neccessary. But is this a realistic scenario? All function entries are aligned 8 bytes. Are there arm64 implementations out there that fetch only 4 bytes and give a chance to mess with the 2nd 4 bytes? You at arm.com should know, and I won't be surprised if the answer is a weird "yes". Or maybe it's just another erratum lurking somewhere... My point is: those 2 insn will _never_ be split by any alignment boundary > 8; does that mean anything, have you considered this? > > I wonder if we could solve that by patching the kernel at build-time, to > > add the MOV X9, X30 in place of the first NOP. If we were to do that, we > > could also update the addresses to pooint at the second NOP, simplifying > > the changes to the runtime code. > > You can also patch it at boot up when there's only one CPU running, and > interrupts are disabled. May I remind about possible performance hits? Even the NOPs had a tiny impact on certain in-order implementations. I'd rather switch between the mov and a "b +2". Torsten _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel