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 v7 changes: - updated snapshot info output format - fixed qcow2 snapshot-related tests 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 (10): replay: provide an accessor for rr filename 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 (4): replay: don't record interrupt poll qcow2: introduce icount field for snapshots 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 | 334 +++++++++++++++++++++++++++++++++ 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/261 | 19 +- tests/qemu-iotests/261.out | 51 +++-- tests/qemu-iotests/267.out | 48 ++--- 33 files changed, 1086 insertions(+), 89 deletions(-) create mode 100644 qapi/replay.json create mode 100644 replay/replay-debugging.c create mode 100644 tests/acceptance/reverse_debugging.py -- Pavel Dovgalyuk
