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Tue, 06 Apr 2021 08:01:05 -0700 (PDT) Received: from zen.linaroharston ([51.148.130.216]) by smtp.gmail.com with ESMTPSA id q79sm3398247wme.11.2021.04.06.08.00.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 06 Apr 2021 08:00:51 -0700 (PDT) Received: from zen.lan (localhost [127.0.0.1]) by zen.linaroharston (Postfix) with ESMTP id D33171FF98; Tue, 6 Apr 2021 16:00:42 +0100 (BST) From: =?UTF-8?q?Alex=20Benn=C3=A9e?= To: peter.maydell@linaro.org Subject: [PULL 09/11] docs/system/gdb.rst: Document how to debug multicore machines Date: Tue, 6 Apr 2021 16:00:39 +0100 Message-Id: <20210406150041.28753-10-alex.bennee@linaro.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20210406150041.28753-1-alex.bennee@linaro.org> References: <20210406150041.28753-1-alex.bennee@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=2a00:1450:4864:20::335; envelope-from=alex.bennee@linaro.org; helo=mail-wm1-x335.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham 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: =?UTF-8?q?Alex=20Benn=C3=A9e?= , qemu-devel@nongnu.org, =?UTF-8?q?Philippe=20Mathieu-Daud=C3=A9?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" From: Peter Maydell Document how multicore machines appear to GDB when debugged via the debug stub. This is particularly non-intuitive for the "multiple heterogenous clusters" case, but unfortunately as far as I know there is no way with the remote protocol for the stub to tell gdb "I have 2 inferiors, please connect to both", so the user must set it all up manually. Signed-off-by: Peter Maydell Signed-off-by: Alex Bennée Reviewed-by: Philippe Mathieu-Daudé Message-Id: <20210325175023.13838-3-peter.maydell@linaro.org> Message-Id: <20210401102530.12030-10-alex.bennee@linaro.org> diff --git a/docs/system/gdb.rst b/docs/system/gdb.rst index 0bb1bedf1b..144d083df3 100644 --- a/docs/system/gdb.rst +++ b/docs/system/gdb.rst @@ -45,6 +45,61 @@ Here are some useful tips in order to use gdb on system code: 3. Use ``set architecture i8086`` to dump 16 bit code. Then use ``x/10i $cs*16+$eip`` to dump the code at the PC position. +Debugging multicore machines +============================ + +GDB's abstraction for debugging targets with multiple possible +parallel flows of execution is a two layer one: it supports multiple +"inferiors", each of which can have multiple "threads". When the QEMU +machine has more than one CPU, QEMU exposes each CPU cluster as a +separate "inferior", where each CPU within the cluster is a separate +"thread". Most QEMU machine types have identical CPUs, so there is a +single cluster which has all the CPUs in it. A few machine types are +heterogenous and have multiple clusters: for example the ``sifive_u`` +machine has a cluster with one E51 core and a second cluster with four +U54 cores. Here the E51 is the only thread in the first inferior, and +the U54 cores are all threads in the second inferior. + +When you connect gdb to the gdbstub, it will automatically +connect to the first inferior; you can display the CPUs in this +cluster using the gdb ``info thread`` command, and switch between +them using gdb's usual thread-management commands. + +For multi-cluster machines, unfortunately gdb does not by default +handle multiple inferiors, and so you have to explicitly connect +to them. First, you must connect with the ``extended-remote`` +protocol, not ``remote``:: + + (gdb) target extended-remote localhost:1234 + +Once connected, gdb will have a single inferior, for the +first cluster. You need to create inferiors for the other +clusters and attach to them, like this:: + + (gdb) add-inferior + Added inferior 2 + (gdb) inferior 2 + [Switching to inferior 2 [] ()] + (gdb) attach 2 + Attaching to process 2 + warning: No executable has been specified and target does not support + determining executable automatically. Try using the "file" command. + 0x00000000 in ?? () + +Once you've done this, ``info threads`` will show CPUs in +all the clusters you have attached to:: + + (gdb) info threads + Id Target Id Frame + 1.1 Thread 1.1 (cortex-m33-arm-cpu cpu [running]) 0x00000000 in ?? () + * 2.1 Thread 2.2 (cortex-m33-arm-cpu cpu [halted ]) 0x00000000 in ?? () + +You probably also want to set gdb to ``schedule-multiple`` mode, +so that when you tell gdb to ``continue`` it resumes all CPUs, +not just those in the cluster you are currently working on:: + + (gdb) set schedule-multiple on + Advanced debugging options ========================== -- 2.20.1