5067946775
The PhyMemMode gdb extension command was missing from the gdb.rst document. Signed-off-by: Jon Doron <arilou@gmail.com> Signed-off-by: Alex Bennée <alex.bennee@linaro.org> Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org> Message-Id: <20200601171609.1665397-1-arilou@gmail.com> Message-Id: <20200709141327.14631-4-alex.bennee@linaro.org>
110 lines
3.6 KiB
ReStructuredText
110 lines
3.6 KiB
ReStructuredText
.. _gdb_005fusage:
|
|
|
|
GDB usage
|
|
---------
|
|
|
|
QEMU supports working with gdb via gdb's remote-connection facility
|
|
(the "gdbstub"). This allows you to debug guest code in the same
|
|
way that you might with a low-level debug facility like JTAG
|
|
on real hardware. You can stop and start the virtual machine,
|
|
examine state like registers and memory, and set breakpoints and
|
|
watchpoints.
|
|
|
|
In order to use gdb, launch QEMU with the ``-s`` and ``-S`` options.
|
|
The ``-s`` option will make QEMU listen for an incoming connection
|
|
from gdb on TCP port 1234, and ``-S`` will make QEMU not start the
|
|
guest until you tell it to from gdb. (If you want to specify which
|
|
TCP port to use or to use something other than TCP for the gdbstub
|
|
connection, use the ``-gdb dev`` option instead of ``-s``.)
|
|
|
|
.. parsed-literal::
|
|
|
|
|qemu_system| -s -S -kernel bzImage -hda rootdisk.img -append "root=/dev/hda"
|
|
|
|
QEMU will launch but will silently wait for gdb to connect.
|
|
|
|
Then launch gdb on the 'vmlinux' executable::
|
|
|
|
> gdb vmlinux
|
|
|
|
In gdb, connect to QEMU::
|
|
|
|
(gdb) target remote localhost:1234
|
|
|
|
Then you can use gdb normally. For example, type 'c' to launch the
|
|
kernel::
|
|
|
|
(gdb) c
|
|
|
|
Here are some useful tips in order to use gdb on system code:
|
|
|
|
1. Use ``info reg`` to display all the CPU registers.
|
|
|
|
2. Use ``x/10i $eip`` to display the code at the PC position.
|
|
|
|
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.
|
|
|
|
Advanced debugging options:
|
|
|
|
The default single stepping behavior is step with the IRQs and timer
|
|
service routines off. It is set this way because when gdb executes a
|
|
single step it expects to advance beyond the current instruction. With
|
|
the IRQs and timer service routines on, a single step might jump into
|
|
the one of the interrupt or exception vectors instead of executing the
|
|
current instruction. This means you may hit the same breakpoint a number
|
|
of times before executing the instruction gdb wants to have executed.
|
|
Because there are rare circumstances where you want to single step into
|
|
an interrupt vector the behavior can be controlled from GDB. There are
|
|
three commands you can query and set the single step behavior:
|
|
|
|
``maintenance packet qqemu.sstepbits``
|
|
This will display the MASK bits used to control the single stepping
|
|
IE:
|
|
|
|
::
|
|
|
|
(gdb) maintenance packet qqemu.sstepbits
|
|
sending: "qqemu.sstepbits"
|
|
received: "ENABLE=1,NOIRQ=2,NOTIMER=4"
|
|
|
|
``maintenance packet qqemu.sstep``
|
|
This will display the current value of the mask used when single
|
|
stepping IE:
|
|
|
|
::
|
|
|
|
(gdb) maintenance packet qqemu.sstep
|
|
sending: "qqemu.sstep"
|
|
received: "0x7"
|
|
|
|
``maintenance packet Qqemu.sstep=HEX_VALUE``
|
|
This will change the single step mask, so if wanted to enable IRQs on
|
|
the single step, but not timers, you would use:
|
|
|
|
::
|
|
|
|
(gdb) maintenance packet Qqemu.sstep=0x5
|
|
sending: "qemu.sstep=0x5"
|
|
received: "OK"
|
|
|
|
|
|
Another feature that QEMU gdbstub provides is to toggle the memory GDB
|
|
works with, by default GDB will show the current process memory respecting
|
|
the virtual address translation.
|
|
|
|
If you want to examine/change the physical memory you can set the gdbstub
|
|
to work with the physical memory rather with the virtual one.
|
|
|
|
The memory mode can be checked by sending the following command:
|
|
|
|
``maintenance packet qqemu.PhyMemMode``
|
|
This will return either 0 or 1, 1 indicates you are currently in the
|
|
physical memory mode.
|
|
|
|
``maintenance packet Qqemu.PhyMemMode:1``
|
|
This will change the memory mode to physical memory.
|
|
|
|
``maintenance packet Qqemu.PhyMemMode:0``
|
|
This will change it back to normal memory mode.
|