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[Qemu-devel] [Bug 1364501] Re: Gdb hangs when trying to single-step afte
[Qemu-devel] [Bug 1364501] Re: Gdb hangs when trying to single-step after an invalid instruction
Wed, 05 Dec 2018 11:06:36 -0000
** Tags added: gdbstub
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Gdb hangs when trying to single-step after an invalid instruction
Status in QEMU:
When using Gdb to remote-debug a program and manually setting its PC
to point to an address containing an invalid instruction and then
doing a single step, Qemu will never return control to the remote Gdb.
For instance, let's say address 0x114 contains an invalid instruction.
On the remote Gdb, we'd do:
(gdb) set $pc = 0x114
After doing that we won't get the (gdb) prompt unless we do a Ctrl-C.
If we do so we'll be left at 0x114 instead of going towards the
exception handler as we should. This happens with stepi, step and
next. If instead of single-stepping we used continue, the program will
proceed into the exception handler as it should.
The reason this is happening is that when Qemu realizes it's about to
translate an instruction it doesn't recognize it'll generate a call to
helper_exception_with_syndrome(), which will register the exception
and then call cpu_loop_exit(). At the same time, because we're doing a
single-step, Qemu will also generate a call to
helper_exception_internal() passing it an EXCP_DEBUG, which lets the
system know it'll give control back to the remote debugger, and it
also ends with a call to cpu_loop_exit(). However, because the
syndrome exception calls cpu_loop_exit() first, the call to the
internal exception won't be reached and Qemu will be stuck in a loop
without returning control to the remote debugger.
What makes this a bit tricky to fix is that we must call
cpu_loop_exit() at the end of helper_exception_with_syndrome(),
otherwise the target exception will go undetected and its handler
won't be excecuted.
Tested on latest head by emulating a Stellaris lm3s6965 board and
running RTEMS 4.11:
$ qemu-system-arm -nographic -s -S -M lm3s6965evb -kernel my_rtems_app
Commit hash in qemu.git: 30eaca3acdf17d7bcbd1213eb149c02037edfb0b
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