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 lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id E8906C27C55 for ; Mon, 10 Jun 2024 20:06:52 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4VyjWG579nz3cXL for ; Tue, 11 Jun 2024 06:06:50 +1000 (AEST) Authentication-Results: lists.ozlabs.org; dmarc=none (p=none dis=none) header.from=goodmis.org Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=kernel.org (client-ip=2604:1380:40e1:4800::1; helo=sin.source.kernel.org; envelope-from=srs0=wnj5=nm=goodmis.org=rostedt@kernel.org; receiver=lists.ozlabs.org) Received: from sin.source.kernel.org (sin.source.kernel.org [IPv6:2604:1380:40e1:4800::1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4VyjVr5Y54z30Vy for ; Tue, 11 Jun 2024 06:06:28 +1000 (AEST) Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sin.source.kernel.org (Postfix) with ESMTP id 3B379CE181F; Mon, 10 Jun 2024 20:06:20 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 924D5C2BBFC; Mon, 10 Jun 2024 20:06:18 +0000 (UTC) Date: Mon, 10 Jun 2024 16:06:32 -0400 From: Steven Rostedt To: Naveen N Rao Subject: Re: [RFC PATCH v2 3/5] powerpc/ftrace: Unify 32-bit and 64-bit ftrace entry code Message-ID: <20240610160632.089b6c04@gandalf.local.home> In-Reply-To: References: X-Mailer: Claws Mail 3.20.0git84 (GTK+ 2.24.33; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , Masahiro Yamada , Nicholas Piggin , linuxppc-dev@lists.ozlabs.org, Masami Hiramatsu Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Mon, 10 Jun 2024 14:08:16 +0530 Naveen N Rao wrote: > On 32-bit powerpc, gcc generates a three instruction sequence for > function profiling: > mflr r0 > stw r0, 4(r1) > bl _mcount > > On kernel boot, the call to _mcount() is nop-ed out, to be patched back > in when ftrace is actually enabled. The 'stw' instruction therefore is > not necessary unless ftrace is enabled. Nop it out during ftrace init. > > When ftrace is enabled, we want the 'stw' so that stack unwinding works > properly. Perform the same within the ftrace handler, similar to 64-bit > powerpc. > > For 64-bit powerpc, early versions of gcc used to emit a three > instruction sequence for function profiling (with -mprofile-kernel) with > a 'std' instruction to mimic the 'stw' above. Address that scenario also > by nop-ing out the 'std' instruction during ftrace init. > > Signed-off-by: Naveen N Rao Isn't there still the race that there's a preemption between the: stw r0, 4(r1) and bl _mcount And if this breaks stack unwinding, couldn't this cause an issue for live kernel patching? I know it's very unlikely, but in theory, I think the race exists. -- Steve