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 <peter.maydell@linaro.org> Signed-off-by: Alex Bennée <alex.bennee@linaro.org> Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org> Message-Id: <20210325175023.13838-3-peter.maydell@linaro.org> Message-Id: <20210401102530.12030-10-alex.bennee@linaro.org>
171 lines
6 KiB
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171 lines
6 KiB
ReStructuredText
.. _GDB usage:
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GDB usage
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---------
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QEMU supports working with gdb via gdb's remote-connection facility
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(the "gdbstub"). This allows you to debug guest code in the same
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way that you might with a low-level debug facility like JTAG
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on real hardware. You can stop and start the virtual machine,
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examine state like registers and memory, and set breakpoints and
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watchpoints.
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In order to use gdb, launch QEMU with the ``-s`` and ``-S`` options.
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The ``-s`` option will make QEMU listen for an incoming connection
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from gdb on TCP port 1234, and ``-S`` will make QEMU not start the
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guest until you tell it to from gdb. (If you want to specify which
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TCP port to use or to use something other than TCP for the gdbstub
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connection, use the ``-gdb dev`` option instead of ``-s``.)
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.. parsed-literal::
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|qemu_system| -s -S -kernel bzImage -hda rootdisk.img -append "root=/dev/hda"
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QEMU will launch but will silently wait for gdb to connect.
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Then launch gdb on the 'vmlinux' executable::
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> gdb vmlinux
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In gdb, connect to QEMU::
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(gdb) target remote localhost:1234
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Then you can use gdb normally. For example, type 'c' to launch the
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kernel::
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(gdb) c
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Here are some useful tips in order to use gdb on system code:
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1. Use ``info reg`` to display all the CPU registers.
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2. Use ``x/10i $eip`` to display the code at the PC position.
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3. Use ``set architecture i8086`` to dump 16 bit code. Then use
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``x/10i $cs*16+$eip`` to dump the code at the PC position.
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Debugging multicore machines
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============================
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GDB's abstraction for debugging targets with multiple possible
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parallel flows of execution is a two layer one: it supports multiple
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"inferiors", each of which can have multiple "threads". When the QEMU
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machine has more than one CPU, QEMU exposes each CPU cluster as a
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separate "inferior", where each CPU within the cluster is a separate
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"thread". Most QEMU machine types have identical CPUs, so there is a
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single cluster which has all the CPUs in it. A few machine types are
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heterogenous and have multiple clusters: for example the ``sifive_u``
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machine has a cluster with one E51 core and a second cluster with four
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U54 cores. Here the E51 is the only thread in the first inferior, and
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the U54 cores are all threads in the second inferior.
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When you connect gdb to the gdbstub, it will automatically
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connect to the first inferior; you can display the CPUs in this
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cluster using the gdb ``info thread`` command, and switch between
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them using gdb's usual thread-management commands.
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For multi-cluster machines, unfortunately gdb does not by default
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handle multiple inferiors, and so you have to explicitly connect
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to them. First, you must connect with the ``extended-remote``
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protocol, not ``remote``::
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(gdb) target extended-remote localhost:1234
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Once connected, gdb will have a single inferior, for the
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first cluster. You need to create inferiors for the other
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clusters and attach to them, like this::
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(gdb) add-inferior
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Added inferior 2
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(gdb) inferior 2
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[Switching to inferior 2 [<null>] (<noexec>)]
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(gdb) attach 2
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Attaching to process 2
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warning: No executable has been specified and target does not support
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determining executable automatically. Try using the "file" command.
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0x00000000 in ?? ()
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Once you've done this, ``info threads`` will show CPUs in
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all the clusters you have attached to::
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(gdb) info threads
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Id Target Id Frame
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1.1 Thread 1.1 (cortex-m33-arm-cpu cpu [running]) 0x00000000 in ?? ()
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* 2.1 Thread 2.2 (cortex-m33-arm-cpu cpu [halted ]) 0x00000000 in ?? ()
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You probably also want to set gdb to ``schedule-multiple`` mode,
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so that when you tell gdb to ``continue`` it resumes all CPUs,
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not just those in the cluster you are currently working on::
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(gdb) set schedule-multiple on
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Advanced debugging options
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==========================
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Changing single-stepping behaviour
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The default single stepping behavior is step with the IRQs and timer
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service routines off. It is set this way because when gdb executes a
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single step it expects to advance beyond the current instruction. With
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the IRQs and timer service routines on, a single step might jump into
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the one of the interrupt or exception vectors instead of executing the
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current instruction. This means you may hit the same breakpoint a number
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of times before executing the instruction gdb wants to have executed.
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Because there are rare circumstances where you want to single step into
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an interrupt vector the behavior can be controlled from GDB. There are
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three commands you can query and set the single step behavior:
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``maintenance packet qqemu.sstepbits``
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This will display the MASK bits used to control the single stepping
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IE:
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::
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(gdb) maintenance packet qqemu.sstepbits
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sending: "qqemu.sstepbits"
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received: "ENABLE=1,NOIRQ=2,NOTIMER=4"
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``maintenance packet qqemu.sstep``
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This will display the current value of the mask used when single
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stepping IE:
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::
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(gdb) maintenance packet qqemu.sstep
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sending: "qqemu.sstep"
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received: "0x7"
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``maintenance packet Qqemu.sstep=HEX_VALUE``
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This will change the single step mask, so if wanted to enable IRQs on
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the single step, but not timers, you would use:
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::
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(gdb) maintenance packet Qqemu.sstep=0x5
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sending: "qemu.sstep=0x5"
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received: "OK"
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Examining physical memory
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^^^^^^^^^^^^^^^^^^^^^^^^^
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Another feature that QEMU gdbstub provides is to toggle the memory GDB
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works with, by default GDB will show the current process memory respecting
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the virtual address translation.
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If you want to examine/change the physical memory you can set the gdbstub
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to work with the physical memory rather with the virtual one.
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The memory mode can be checked by sending the following command:
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``maintenance packet qqemu.PhyMemMode``
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This will return either 0 or 1, 1 indicates you are currently in the
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physical memory mode.
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``maintenance packet Qqemu.PhyMemMode:1``
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This will change the memory mode to physical memory.
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``maintenance packet Qqemu.PhyMemMode:0``
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This will change it back to normal memory mode.
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