Re: [PATCH v6 00/14] Reverse debugging

[Paolo Bonzini]

On 29/09/20 13:01, Pavel Dovgalyuk wrote:
> 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
>  - avocado-based acceptance tests for reverse debugging
> 
> The patches are available in the repository:
> https://github.com/ispras/qemu/tree/rr-200901

Hi Pavel,

I'm still seeing failures in "make check-block":

https://gitlab.com/bonzini/qemu/-/jobs/769653852

Paolo

> v6 changes:
>  - removed passing err variable without checking it's value after
> v5 changes:
>  - disabled reverse debugging tests for gitlab-based testing
>    due to the unidentified timeout problem
> v4 changes:
>  - added VM snapshot creation on gdb connect (suggested by Alex Bennée)
>  - removed useless calls to error_free
>  - updated poll interrupt processing
>  - minor changes
> v3 changes:
>  - rebased to support the new build system
>  - bumped avocado framework version for using fixed remote gdb client
> v2 changes:
>  - rebased to the latest upstream version
>  - fixed replaying of the POLL interrupts after the latest debug changes
> 
> ---
> 
> Pavel Dovgaluk (11):
>       replay: provide an accessor for rr filename
>       qcow2: 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
>       tests/acceptance: add reverse debugging test
> 
> Pavel Dovgalyuk (3):
>       replay: don't record interrupt poll
>       migration: introduce icount field for snapshots
>       replay: create temporary snapshot at debugger connection
> 
> 
>  MAINTAINERS                           |    2 
>  accel/tcg/cpu-exec.c                  |   21 ++
>  accel/tcg/translator.c                |    1 
>  block/qapi.c                          |   18 +-
>  block/qcow2-snapshot.c                |    9 +
>  block/qcow2.h                         |    3 
>  blockdev.c                            |   10 +
>  docs/interop/qcow2.txt                |    5 
>  docs/replay.txt                       |   46 +++++
>  exec.c                                |    8 +
>  gdbstub.c                             |   64 ++++++
>  hmp-commands-info.hx                  |   11 +
>  hmp-commands.hx                       |   50 +++++
>  include/block/snapshot.h              |    1 
>  include/monitor/hmp.h                 |    4 
>  include/sysemu/replay.h               |   26 +++
>  migration/savevm.c                    |   17 +-
>  qapi/block-core.json                  |   11 +
>  qapi/meson.build                      |    1 
>  qapi/misc.json                        |   18 --
>  qapi/qapi-schema.json                 |    1 
>  qapi/replay.json                      |  121 ++++++++++++
>  replay/meson.build                    |    1 
>  replay/replay-debugging.c             |  332 
> +++++++++++++++++++++++++++++++++
>  replay/replay-events.c                |    4 
>  replay/replay-internal.h              |    6 -
>  replay/replay.c                       |   22 ++
>  softmmu/cpus.c                        |   19 ++
>  stubs/replay.c                        |   15 +
>  tests/acceptance/reverse_debugging.py |  208 +++++++++++++++++++++
>  tests/qemu-iotests/267.out            |   48 ++---
>  31 files changed, 1039 insertions(+), 64 deletions(-)
>  create mode 100644 qapi/replay.json
>  create mode 100644 replay/replay-debugging.c
>  create mode 100644 tests/acceptance/reverse_debugging.py
> 
> --
> Pavel Dovgalyuk
>