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=-2.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 73341C43331 for ; Fri, 3 Apr 2020 08:12:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3DFFE206F8 for ; Fri, 3 Apr 2020 08:12:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="UEVfaAhq" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2390472AbgDCIMC (ORCPT ); Fri, 3 Apr 2020 04:12:02 -0400 Received: from us-smtp-2.mimecast.com ([205.139.110.61]:48814 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727431AbgDCIMB (ORCPT ); Fri, 3 Apr 2020 04:12:01 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1585901520; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=8PHaNpy+983QDwfaUcTgQtvDFchwmBDQqNsuXGeebd8=; b=UEVfaAhqFvAduemYCfJ1yUlJhGUa/UpvV/1o/cb3lElR8Rd9aL82SzguQGggj79rLrpBaq qygMxalPuRY12qodDoTrnAaPnZMsk9wwYwf0gsKtGi/8ISzFxBQti/0Uldv6dSUiuqHa+z I9bXOza5fJMfWsOIOdN2qzpqQONvZa8= Received: from mail-wm1-f70.google.com (mail-wm1-f70.google.com [209.85.128.70]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-498--aXC1AWENeaU6P-ES7T-Kw-1; Fri, 03 Apr 2020 04:11:59 -0400 X-MC-Unique: -aXC1AWENeaU6P-ES7T-Kw-1 Received: by mail-wm1-f70.google.com with SMTP id f128so1134174wmf.2 for ; Fri, 03 Apr 2020 01:11:59 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=8PHaNpy+983QDwfaUcTgQtvDFchwmBDQqNsuXGeebd8=; b=RlQs3w26oVG6Zb9Ty/9TLCkQgWnyrdJzPWgtFwGt0QursuWgCVtnN2AA6MIaN4n/8d qNR731+QGns79tLRfxTiDoGYXVRYZWs4dO0Sv5KPMgIM6xiiRbU6N0sU6dm1CdHqpE1J mkmiWd9GWM6KQ/oB3HXeDzwoqobhNQzMy/DBK2EgYngKAXVHr24ThXia5oGiv42tR5qR lRbddfWy0l+hHeUZufSAmr7pyBsqtqa+umyTOa+vs6Gvj+Hy2jpUFFDKI0KXv/5mR5yJ P4eTAmlpCqTpaOUVtJeh0IIRDO+VplcLFdTunlf0ANXt4t52PFyUcospcBtnRNGvTMxZ CzHQ== X-Gm-Message-State: AGi0Pua7FRB+XoxrH0lHD1+dEV5FDiuEP45Aqd4DDLvuagmCPPr6ytjH 9tpT2yQXXEhlVwMLVWw72Z+NN+4PXal7Doe9uW1xmyfwoWTjHUO8yiPKq1A9IcgnBxqncA6/1pN sIKlcPOrVMb+CoeVdrH0kEDYJ X-Received: by 2002:a1c:3b89:: with SMTP id i131mr7421838wma.35.1585901518312; Fri, 03 Apr 2020 01:11:58 -0700 (PDT) X-Google-Smtp-Source: APiQypLS2c3t7i5T//sRgt+XOqdSZOMSgeSIcbtsg6NJaDUMR/JgDrbvYFytbDZ80B91oF0tToS3Gw== X-Received: by 2002:a1c:3b89:: with SMTP id i131mr7421765wma.35.1585901517379; Fri, 03 Apr 2020 01:11:57 -0700 (PDT) Received: from ?IPv6:2a01:cb14:58d:8400:ecf6:58e2:9c06:a308? ([2a01:cb14:58d:8400:ecf6:58e2:9c06:a308]) by smtp.gmail.com with ESMTPSA id a12sm9146807wmj.22.2020.04.03.01.11.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 03 Apr 2020 01:11:56 -0700 (PDT) Subject: Re: [PATCH 4/7] objtool: Add support for return trampoline call To: Peter Zijlstra Cc: Alexandre Chartre , x86@kernel.org, linux-kernel@vger.kernel.org, jpoimboe@redhat.com, tglx@linutronix.de References: <20200402082220.808-1-alexandre.chartre@oracle.com> <20200402082220.808-5-alexandre.chartre@oracle.com> <20200402154022.GG20730@hirez.programming.kicks-ass.net> From: Julien Thierry Message-ID: Date: Fri, 3 Apr 2020 09:11:55 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <20200402154022.GG20730@hirez.programming.kicks-ass.net> 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 On 4/2/20 4:40 PM, Peter Zijlstra wrote: > On Thu, Apr 02, 2020 at 04:31:05PM +0100, Julien Thierry wrote: >> My understanding is that once you find an intra-function call, either you >> hit a return, ending the branch, so the return should undo the modification >> the intra-function call did (whether is it a retpoline return or not). >> Otherwise, the intra-function call branch will need to reach an end in some >> way (e.g. hitting a CONTEXT_SWITCH instruction, calling a >> dead_end_function). >> >> Am I missing something? > > The thing is basically doing: > > mov $n, cx > 1: call 2f > 2: dec cx > jnz 1b > add 8*n, sp > > So it does N calls to self, then subtracts N words from the stack. > > The reason being that the CPU has a return-stack-buffer for predicting > returns, and call/ret being naturally paired, that works. The above > is a software flush of the RSB. > Ah, lovely... Maybe that's where SAVE/RESTORE unwind hints could be nice ;) . Otherwise, I don't really have a good suggestion for this... -- Julien Thierry