RE: [for-5.0 PATCH 00/11] Support for reverse debugging with GDB

["Pavel Dovgalyuk" <dovgaluk@ispras.ru>]

> From: Alex Bennée [mailto:alex.bennee@linaro.org]
> Pavel Dovgalyuk <pavel.dovgaluk@gmail.com> writes:
> 
> > GDB remote protocol supports reverse debugging of the targets.
> > It includes 'reverse step' and 'reverse continue' operations.
> > The first one finds the previous step of the execution,
> > and the second one is intended to stop at the last breakpoint that
> > would happen when the program is executed normally.
> >
> > Reverse debugging is possible in the replay mode, when at least
> > one snapshot was created at the record or replay phase.
> > QEMU can use these snapshots for travelling back in time with GDB.
> >
> > Running the execution in replay mode allows using GDB reverse debugging
> > commands:
> >  - reverse-stepi (or rsi): Steps one instruction to the past.
> >    QEMU loads on of the prior snapshots and proceeds to the desired
> >    instruction forward. When that step is reaches, execution stops.
> >  - reverse-continue (or rc): Runs execution "backwards".
> >    QEMU tries to find breakpoint or watchpoint by loaded prior snapshot
> >    and replaying the execution. Then QEMU loads snapshots again and
> >    replays to the latest breakpoint. When there are no breakpoints in
> >    the examined section of the execution, QEMU finds one more snapshot
> >    and tries again. After the first snapshot is processed, execution
> >    stops at this snapshot.
> >
> > The set of patches include the following modifications:
> >  - gdbstub update for reverse debugging support
> >  - functions that automatically perform reverse step and reverse
> >    continue operations
> >  - hmp/qmp commands for manipulating the replay process
> >  - improvement of the snapshotting for saving the execution step
> >    in the snapshot parameters
> >
> > The patches are available in the repository:
> > https://github.com/ispras/qemu/tree/rr-191223
> 
> So I tried with your additional patch. Launching QEMU as:
> 
>   ./aarch64-softmmu//qemu-system-aarch64 -monitor none \
>      -display none -M virt -cpu max -display none \
>      -semihosting-config enable=on \
>      -kernel ./tests/tcg/aarch64-softmmu/memory \
>      -icount shift=5,rr=replay,rrfile=record.bin \
>      -s -S -d trace:gdbstub\*
> 
> And gdb:
> 
>   gdb tests/tcg/aarch64-softmmu/memory \
>     -ex "target remote localhost:1234"
> 
> I get the following log:
> 
>   (gdb) x/3i $pc
>   => 0x400037b0 <__start>:        adr     x0, 0x40003000 <vector_table>
>      0x400037b4 <__start+4>:      msr     vbar_el1, x0
>      0x400037b8 <__start+8>:      adrp    x0, 0x40200000
>   (gdb) p/x $x0
>   $1 = 0x0
>   (gdb) si
>   92              msr     vbar_el1, x0
>   (gdb) p/x $x0
>   $2 = 0x40003000
>   (gdb) rsi
>   warning: Remote failure reply: E14
> 
>   Program stopped.
>   __start () at /home/alex.bennee/lsrc/qemu.git/tests/tcg/aarch64/system/boot.S:92
>   92              msr     vbar_el1, x0
>   (gdb) p/x $x0
>   $3 = 0x40003000
> 
> So it doesn't seem to be working.

That's ok, you'll need at least one VM snapshot available to recover the initial VM state.
Try changing the command lines in the following way:

First, create empty.qcow2 which will be used for saving the snapshots.
Then record with initial snapshot and attached empty.qcow2:

   ./aarch64-softmmu//qemu-system-aarch64 -monitor none \
      -display none -M virt -cpu max \
      -kernel ./tests/tcg/aarch64-softmmu/memory \
      -icount shift=5,rr=record,rrfile=record.bin,rrsnapshot=init \
      -drive file=empty.qcow2

After that you can replay the execution with the support of reverse debugging:

   ./aarch64-softmmu//qemu-system-aarch64 -monitor none \
      -display none -M virt -cpu max \
      -kernel ./tests/tcg/aarch64-softmmu/memory \
      -icount shift=5,rr=replay,rrfile=record.bin,rrsnapshot=init \
      -drive file=empty.qcow2

I removed "-semihosting-config enable=on", because I'm not sure what is it for.
It may break the replay, because there is no implementation of
semihosting input record/replay.

Pavel Dovgalyuk