From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 126073 invoked by alias); 11 Apr 2018 11:19:52 -0000 Mailing-List: contact gdb-patches-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-patches-owner@sourceware.org Received: (qmail 126058 invoked by uid 89); 11 Apr 2018 11:19:52 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-2.8 required=5.0 tests=AWL,BAYES_00 autolearn=ham version=3.3.2 spammy=Want X-HELO: mx1.redhat.com Received: from mx3-rdu2.redhat.com (HELO mx1.redhat.com) (66.187.233.73) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Wed, 11 Apr 2018 11:19:50 +0000 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 5E9EC406804F; Wed, 11 Apr 2018 11:19:49 +0000 (UTC) Received: from [127.0.0.1] (ovpn04.gateway.prod.ext.ams2.redhat.com [10.39.146.4]) by smtp.corp.redhat.com (Postfix) with ESMTP id BB96810B0F33; Wed, 11 Apr 2018 11:19:48 +0000 (UTC) Subject: Re: [PATCH 0/3 v3] [AArch64] Support tagged pointer To: Omair Javaid References: <1512727471-30745-1-git-send-email-yao.qi@linaro.org> <5429b7f0-ee91-67f4-3b15-f5de9aa06389@redhat.com> Cc: Yao Qi , GDB Patches From: Pedro Alves Message-ID: <5e21c13b-9261-f947-e06c-dad9568278bf@redhat.com> Date: Wed, 11 Apr 2018 11:19:00 -0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit X-SW-Source: 2018-04/txt/msg00213.txt.bz2 On 04/11/2018 12:12 PM, Omair Javaid wrote: > On 11 April 2018 at 15:13, Pedro Alves > wrote: > > On 04/11/2018 01:15 AM, Omair Javaid wrote: > > > This patch has broken kernel debugging using kgdb and openOCD. > > OOC, can you qualify this a bit more, please? > > Does the kernel use the high bits for something? > > > We can safely assume that top byte is 0 in case of user address space on linux because it enables tagging support but not for kernel address space. > > According to linux memory layout of AArch64 given here: https://www.kernel.org/doc/Documentation/arm64/memory.txt > > "User addresses have bits 63:48 set to 0 while the kernel addresses have ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > the same bits set to 1. TTBRx selection is given by bit 63 of the ^^^^^^^^^^^^^^^^^^^^^^^^^ > virtual address." Ah, that's clear as day now. > > According to kernel document on tagged pointer support in AArch64 given here: https://www.kernel.org/doc/Documentation/arm64/tagged-pointers.txt > > The kernel configures the translation tables so that translations made > via TTBR0 (i.e. userspace mappings) have the top byte (bits 63:56) of > the virtual address ignored by the translation hardware. This frees up > this byte for application use. > > With set_gdbarch_significant_addr_bit applied to aarch64-tdep following happens when gdb tries reading kernel address space memory: > > query the 0xffffffc000092698 memory data, GDB sent "m00ffffc000092698,4" instead of "mffffffc000092698,4" > OK, that makes a lot more sense now. The above is the perfect info to be included in a git commit log. Want to submit a patch? Thanks, Pedro Alves