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

["Alex Bennée" <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.

>
> ---
>
> Pavel Dovgaluk (11):
>       replay: provide an accessor for rr filename
>       qcow2: introduce icount field for snapshots
>       migration: introduce icount field for snapshots
>       qapi: introduce replay.json for record/replay-related stuff
>       replay: introduce info hmp/qmp command
>       replay: introduce breakpoint at the specified step
>       replay: implement replay-seek command
>       replay: flush rr queue before loading the vmstate
>       gdbstub: add reverse step support in replay mode
>       gdbstub: add reverse continue support in replay mode
>       replay: describe reverse debugging in docs/replay.txt
>
>
>  MAINTAINERS               |    1 
>  accel/tcg/translator.c    |    1 
>  block/qapi.c              |   18 ++
>  block/qcow2-snapshot.c    |    9 +
>  block/qcow2.h             |    3 
>  blockdev.c                |   10 +
>  cpus.c                    |   19 ++-
>  docs/interop/qcow2.txt    |    4 +
>  docs/replay.txt           |   33 +++++
>  exec.c                    |    8 +
>  gdbstub.c                 |   64 ++++++++-
>  hmp-commands-info.hx      |   14 ++
>  hmp-commands.hx           |   53 +++++++
>  include/block/snapshot.h  |    1 
>  include/monitor/hmp.h     |    4 +
>  include/sysemu/replay.h   |   24 +++
>  migration/savevm.c        |   17 ++
>  qapi/Makefile.objs        |    2 
>  qapi/block-core.json      |    8 +
>  qapi/block.json           |    3 
>  qapi/misc.json            |   18 --
>  qapi/qapi-schema.json     |    1 
>  qapi/replay.json          |  121 +++++++++++++++++
>  replay/Makefile.objs      |    1 
>  replay/replay-debugging.c |  325 +++++++++++++++++++++++++++++++++++++++++++++
>  replay/replay-internal.h  |    6 +
>  replay/replay.c           |   22 +++
>  stubs/replay.c            |   10 +
>  28 files changed, 761 insertions(+), 39 deletions(-)
>  create mode 100644 qapi/replay.json
>  create mode 100644 replay/replay-debugging.c


-- 
Alex Bennée