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[83.52.55.163]) by smtp.gmail.com with ESMTPSA id c25sm13628323wmr.15.2021.08.23.08.51.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 23 Aug 2021 08:51:04 -0700 (PDT) Subject: Re: [PATCH 0/3] gdbstub: add support for switchable endianness To: Peter Maydell References: <20210823142004.17935-1-changbin.du@gmail.com> <7523c6ad-52cd-0b20-b09d-01bd537edbb3@redhat.com> From: =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Message-ID: <565f9ab6-7571-3c26-5e57-8b2a49a9e7b1@redhat.com> Date: Mon, 23 Aug 2021 17:51:03 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=philmd@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=philmd@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -55 X-Spam_score: -5.6 X-Spam_bar: ----- X-Spam_report: (-5.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.743, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-2.023, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "open list:RISC-V" , Bin Meng , QEMU Developers , qemu-arm , Alistair Francis , Paolo Bonzini , =?UTF-8?Q?Alex_Benn=c3=a9e?= , Palmer Dabbelt , Changbin Du Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 8/23/21 5:30 PM, Peter Maydell wrote: > On Mon, 23 Aug 2021 at 16:21, Philippe Mathieu-Daudé wrote: >> >> On 8/23/21 4:20 PM, Changbin Du wrote: >>> To resolve the issue to debug switchable targets, this serias introduces >>> basic infrastructure for gdbstub and enable support for ARM and RISC-V >>> targets. >>> >>> For example, now there is no problem to debug an big-enadian aarch64 target >>> on x86 host. >>> >>> $ qemu-system-aarch64 -gdb tcp::1234,endianness=big ... >> >> I don't understand why you need all that. >> Maybe you aren't using gdb-multiarch? >> >> You can install it or start it via QEMU Debian Docker image: >> >> $ docker run -it --rm -v /tmp:/tmp -u $UID --network=host \ >> registry.gitlab.com/qemu-project/qemu/qemu/debian10 \ >> gdb-multiarch -q \ >> --ex 'set architecture aarch64' \ >> --ex 'set endian big' >> The target architecture is assumed to be aarch64 >> The target is assumed to be big endian >> (gdb) target remote 172.17.0.1:1234 > > I don't think that will help, because an AArch64 CPU (at least > in the boards we model) will always start up in little-endian, > and our gdbstub will always transfer register data etc in > little-endian order, because gdb cannot cope with a target that > isn't always the same endianness. Fixing this requires gdb > changes to be more capable of handling dynamic target changes > (this would also help with eg debugging across 32<->64 bit switches); > as I understand it that gdb work would be pretty significant, > and at least for aarch64 pretty much nobody cares about > big-endian, so nobody's got round to doing it yet. > > Our target/ppc/gdbstub.c code takes a different tack: it > always sends register data in the same order the CPU is > currently in, which has a different set of cases when it > goes wrong. I remember having tested the 'setend be' instruction (from https://github.com/pcrost/arm-be-test) 3 years ago using it. Connected as little endian, set breakpoint, on BP hit disconnect, set big-endian, reconnect, keep single-stepping (in the same gdb session). I doubt anything changed since.