On the MIPS target DSP ASE registers can only be accessed with the
PTRACE_PEEKUSR and PTRACE_POKEUSR `ptrace' requests. With the n32 ABI
these requests only pass 32-bit data quantities, which are narrower than
the width of DSP accumulator registers, which are 64-bit.
Generic code is prepared to transfer registers wider than the `ptrace'
data type by offsetting into the USR address space, by the data width
transferred. That however does not work with the MIPS target, because
of how the API has been defined, where USR register addresses are
actually indices rather than offsets. Consequently given address `a'
using `a + 4' accesses the fourth next register rather than the upper
half of the original register.
With native debugging this causes clobbered register contents, as well
as access failures as locations beyond the available USR space are
addressed:
(gdb) info registers
zero at v0 v1
R0 0000000000000000 0000000000000001 0000000000000001 0000000000000000
a0 a1 a2 a3
R4 0000000010019158 0000000000000000 0000000000000011 0000000010019160
a4 a5 a6 a7
R8 0000000010019160 fffffffffff00000 fffffffffffffff8 0000000000000000
t0 t1 t2 t3
R12 0000000010019150 0000000000000001 0000000000000001 000000000000000f
s0 s1 s2 s3
R16 0000000077ee6f20 0000000010007bb0 0000000000000000 0000000000000000
s4 s5 s6 s7
R20 000000000052e668 000000000052f008 0000000000000000 0000000000000000
t8 t9 k0 k1
R24 0000000000000001 0000000010019010 0000000000000000 0000000000000000
gp sp s8 ra
R28 0000000010020280 000000007fff4c10 000000007fff4c10 0000000010004f48
status lo hi badvaddr
0000000000109cf3 0000000000943efe 000000000000000e 000000001001900c
cause pc
0000000000800024 0000000010004f48
fcsr fir hi1 lo1
0e800000 00f30000 0000000004040404 0101010105050505
hi2 lo2 hi3 lo3
0202020255aa33cc Couldn't read register (#75): Input/output error.
(gdb)
With `gdbserver' this makes debugging impossible due to a fatal failure:
(gdb) target remote :2346
Remote debugging using :2346
Reading symbols from .../sysroot/mips-r2-hard/lib32/ld.so.1...done.
0x77fc3d50 in __start () from .../sysroot/mips-r2-hard/lib32/ld.so.1
(gdb) continue
Continuing.
warning: Remote failure reply: E01
Remote communication error. Target disconnected.: Connection reset by peer.
(gdb)
Correct the problem by marking any register in the MIPS backend whose
width exceeds the width of the `ptrace' data type unavailable for the
purpose of PTRACE_PEEKUSR and PTRACE_POKEUSR requests:
(gdb) info registers
zero at v0 v1
R0 0000000000000000 0000000000000001 0000000000000001 0000000000000000
a0 a1 a2 a3
R4 0000000010019158 0000000000000000 0000000000000011 0000000010019160
a4 a5 a6 a7
R8 0000000010019160 fffffffffff00000 fffffffffffffff8 0000000000000000
t0 t1 t2 t3
R12 0000000010019150 0000000000000001 0000000000000001 000000000000000f
s0 s1 s2 s3
R16 0000000077ee6f20 0000000010007bb0 0000000000000000 0000000000000000
s4 s5 s6 s7
R20 000000000052e5c8 000000000052f008 0000000000000000 0000000000000000
t8 t9 k0 k1
R24 0000000000000001 0000000010019010 0000000000000000 0000000000000000
gp sp s8 ra
R28 0000000010020280 000000007fff4be0 000000007fff4be0 0000000010004f48
status lo hi badvaddr
0000000000109cf3 0000000000943efe 000000000000000e 000000001001900c
cause pc
0000000000800024 0000000010004f48
fcsr fir hi1 lo1
0e800000 00f30000 <unavailable> <unavailable>
hi2 lo2 hi3 lo3
<unavailable> <unavailable> <unavailable> <unavailable>
dspctl restart
55aa33cc 0000000000000000
(gdb)
as there is no way to access full contents of these registers with the
limited API available anyway.
This obviously does not affect general-purpose registers (which use the
PTRACE_GETREGS and PTRACE_SETREGS requests for access) or floating-point
general registers (which use PTRACE_GETFPREGS and PTRACE_SETFPREGS).
And $dspctl, being 32-bit, remains accessible too, which is important
for BPOSGE32 branch decoding in single-stepping.
For DSP accumulator access with the n32 ABI a new `ptrace' API is required
on the kernel side.
gdb/
* mips-linux-nat.c (mips64_linux_register_addr): Return -1 if
the width of the requested register exceeds the width of the
`ptrace' data type.
gdb/gdbserver/
* linux-mips-low.c (mips_cannot_fetch_register): Return 1 if the
width of the requested register exceeds the width of the
`ptrace' data type.
(mips_cannot_store_register): Likewise.
README for GDBserver & GDBreplay
by Stu Grossman and Fred Fish
Introduction:
This is GDBserver, a remote server for Un*x-like systems. It can be used to
control the execution of a program on a target system from a GDB on a different
host. GDB and GDBserver communicate using the standard remote serial protocol
implemented in remote.c, and various *-stub.c files. They communicate via
either a serial line or a TCP connection.
For more information about GDBserver, see the GDB manual.
Usage (server (target) side):
First, you need to have a copy of the program you want to debug put onto
the target system. The program can be stripped to save space if needed, as
GDBserver doesn't care about symbols. All symbol handling is taken care of by
the GDB running on the host system.
To use the server, you log on to the target system, and run the `gdbserver'
program. You must tell it (a) how to communicate with GDB, (b) the name of
your program, and (c) its arguments. The general syntax is:
target> gdbserver COMM PROGRAM [ARGS ...]
For example, using a serial port, you might say:
target> gdbserver /dev/com1 emacs foo.txt
This tells GDBserver to debug emacs with an argument of foo.txt, and to
communicate with GDB via /dev/com1. GDBserver now waits patiently for the
host GDB to communicate with it.
To use a TCP connection, you could say:
target> gdbserver host:2345 emacs foo.txt
This says pretty much the same thing as the last example, except that we are
going to communicate with the host GDB via TCP. The `host:2345' argument means
that we are expecting to see a TCP connection from `host' to local TCP port
2345. (Currently, the `host' part is ignored.) You can choose any number you
want for the port number as long as it does not conflict with any existing TCP
ports on the target system. This same port number must be used in the host
GDBs `target remote' command, which will be described shortly. Note that if
you chose a port number that conflicts with another service, GDBserver will
print an error message and exit.
On some targets, GDBserver can also attach to running programs. This is
accomplished via the --attach argument. The syntax is:
target> gdbserver --attach COMM PID
PID is the process ID of a currently running process. It isn't necessary
to point GDBserver at a binary for the running process.
Usage (host side):
You need an unstripped copy of the target program on your host system, since
GDB needs to examine it's symbol tables and such. Start up GDB as you normally
would, with the target program as the first argument. (You may need to use the
--baud option if the serial line is running at anything except 9600 baud.)
Ie: `gdb TARGET-PROG', or `gdb --baud BAUD TARGET-PROG'. After that, the only
new command you need to know about is `target remote'. It's argument is either
a device name (usually a serial device, like `/dev/ttyb'), or a HOST:PORT
descriptor. For example:
(gdb) target remote /dev/ttyb
communicates with the server via serial line /dev/ttyb, and:
(gdb) target remote the-target:2345
communicates via a TCP connection to port 2345 on host `the-target', where
you previously started up GDBserver with the same port number. Note that for
TCP connections, you must start up GDBserver prior to using the `target remote'
command, otherwise you may get an error that looks something like
`Connection refused'.
Building GDBserver:
The supported targets as of November 2006 are:
arm-*-linux*
bfin-*-uclinux
bfin-*-linux-uclibc
crisv32-*-linux*
cris-*-linux*
i[34567]86-*-cygwin*
i[34567]86-*-linux*
i[34567]86-*-mingw*
ia64-*-linux*
m32r*-*-linux*
m68*-*-linux*
m68*-*-uclinux*
mips*64*-*-linux*
mips*-*-linux*
powerpc[64]-*-linux*
s390[x]-*-linux*
sh-*-linux*
spu*-*-*
x86_64-*-linux*
Configuring GDBserver you should specify the same machine for host and
target (which are the machine that GDBserver is going to run on. This
is not the same as the machine that GDB is going to run on; building
GDBserver automatically as part of building a whole tree of tools does
not currently work if cross-compilation is involved (we don't get the
right CC in the Makefile, to start with)).
Building GDBserver for your target is very straightforward. If you build
GDB natively on a target which GDBserver supports, it will be built
automatically when you build GDB. You can also build just GDBserver:
% mkdir obj
% cd obj
% path-to-gdbserver-sources/configure
% make
If you prefer to cross-compile to your target, then you can also build
GDBserver that way. In a Bourne shell, for example:
% export CC=your-cross-compiler
% path-to-gdbserver-sources/configure your-target-name
% make
Using GDBreplay:
A special hacked down version of GDBserver can be used to replay remote
debug log files created by GDB. Before using the GDB "target" command to
initiate a remote debug session, use "set remotelogfile <filename>" to tell
GDB that you want to make a recording of the serial or tcp session. Note
that when replaying the session, GDB communicates with GDBreplay via tcp,
regardless of whether the original session was via a serial link or tcp.
Once you are done with the remote debug session, start GDBreplay and
tell it the name of the log file and the host and port number that GDB
should connect to (typically the same as the host running GDB):
$ gdbreplay logfile host:port
Then start GDB (preferably in a different screen or window) and use the
"target" command to connect to GDBreplay:
(gdb) target remote host:port
Repeat the same sequence of user commands to GDB that you gave in the
original debug session. GDB should not be able to tell that it is talking
to GDBreplay rather than a real target, all other things being equal. Note
that GDBreplay echos the command lines to stderr, as well as the contents of
the packets it sends and receives. The last command echoed by GDBreplay is
the next command that needs to be typed to GDB to continue the session in
sync with the original session.