Re: [Qemu-devel] [PATCH v6 00/25] Fixing record/replay and adding reverse debugging

[Paolo Bonzini <pbonzini@redhat.com>]

On 12/09/2018 10:17, 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:
>  - fixes of record/replay caused by the QEMU core changes
>  - 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
>  - adding new clock for correct timer events from vnc and slirp
>  - other record/replay fixes
> 
> The patches are available in the repository:
> https://github.com/ispras/qemu/tree/rr-180911
> 
> v6 changes:
>  - rebased to the new version of master
>  - fixed build of linux-user configurations
>  - added new clock for slirp and vnc timers
> 
> v5 changes:
>  - multiple fixes of record/replay bugs appeared after QEMU core update
>  - changed reverse debugging to 'since 3.1'
> 
> v4 changes:
>  - changed 'since 2.13' to 'since 3.0' in json (as suggested by Eric Blake)
> 
> v3 changes:
>  - Fixed PS/2 bug with save/load vm, which caused failures of the replay.
>  - Rebased to the new code base.
>  - Minor fixes.
> 
> v2 changes:
>  - documented reverse debugging
>  - fixed start vmstate loading in record mode
>  - documented qcow2 changes (as suggested by Eric Blake)
>  - made icount SnapshotInfo field optional (as suggested by Eric Blake)
>  - renamed qmp commands (as suggested by Eric Blake)
>  - minor changes
> 
> ---
> 
> Pavel Dovgalyuk (25):
>       block: implement bdrv_snapshot_goto for blkreplay
>       replay: disable default snapshot for record/replay
>       replay: update docs for record/replay with block devices
>       replay: don't drain/flush bdrv queue while RR is working
>       replay: finish record/replay before closing the disks
>       qcow2: introduce icount field for snapshots
>       migration: introduce icount field for snapshots
>       replay: provide and accessor for rr filename
>       replay: introduce info hmp/qmp command
>       replay: introduce breakpoint at the specified step
>       replay: implement replay-seek command to proceed to the desired step
>       replay: flush events when exiting
>       replay: refine replay-time module
>       translator: fix breakpoint processing
>       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
>       replay: allow loading any snapshots before recording
>       replay: wake up vCPU when replaying
>       replay: replay BH for IDE trim operation
>       replay: add BH oneshot event for block layer
>       timer: introduce new virtual clock
>       slirp: fix ipv6 timers
>       ui: fix virtual timers
> 
> 
>  accel/tcg/translator.c    |    9 +
>  block/blkreplay.c         |    8 +
>  block/block-backend.c     |    3 
>  block/io.c                |   22 +++
>  block/qapi.c              |   17 ++-
>  block/qcow2-snapshot.c    |    9 +
>  block/qcow2.h             |    2 
>  blockdev.c                |   10 ++
>  cpus.c                    |   50 +++++---
>  docs/interop/qcow2.txt    |    4 +
>  docs/replay.txt           |   45 +++++++
>  exec.c                    |    6 +
>  gdbstub.c                 |   50 +++++++-
>  hmp-commands-info.hx      |   14 ++
>  hmp-commands.hx           |   30 +++++
>  hmp.h                     |    3 
>  hw/ide/core.c             |    3 
>  include/block/snapshot.h  |    1 
>  include/qemu/timer.h      |    9 +
>  include/sysemu/replay.h   |   26 ++++
>  migration/savevm.c        |   15 +-
>  qapi/block-core.json      |    5 +
>  qapi/block.json           |    3 
>  qapi/misc.json            |   68 +++++++++++
>  replay/Makefile.objs      |    3 
>  replay/replay-debugging.c |  287 +++++++++++++++++++++++++++++++++++++++++++++
>  replay/replay-events.c    |   30 +++--
>  replay/replay-internal.h  |    9 +
>  replay/replay-snapshot.c  |   17 ++-
>  replay/replay-time.c      |   32 ++---
>  replay/replay.c           |   38 ++++++
>  slirp/ip6_icmp.c          |    7 +
>  stubs/Makefile.objs       |    1 
>  stubs/replay-user.c       |    9 +
>  stubs/replay.c            |   10 ++
>  ui/input.c                |    8 +
>  util/qemu-timer.c         |    2 
>  vl.c                      |   18 ++-
>  38 files changed, 791 insertions(+), 92 deletions(-)
>  create mode 100644 replay/replay-debugging.c
>  create mode 100644 stubs/replay-user.c
> 

For now I'm queuing 12, 14, 19, 20 (pending question to you) and 23-25.

Kevin, can you take a look at patches 1-5?  I cannot quite evaluate if 4
has any scary ramifications.

Paolo