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 - other record/replay fixes The patches are available in the repository: https://github.com/ispras/qemu/tree/rr-190117 v10 changes: - added patch for correct deadline calculation with external timers - updated icount-related documentation in json files (suggested by Markus Armbruster) - fixed replay shutdown - renamed some functions and variables to make them consistent with the documentation and displayed messages - minor changes v9 changes: - moved rr qapi stuff to the separate file (suggested by Markus Armbruster) - minor coding style fixes v8 changes: - rebased to the new master - added missing fix for prior rr patch - updated 'since' version number in json-related patches v7 changes: - rebased to the new master with upstreamed patches from the series - several improvements in hmp/qmp commands handling (suggested by Markus Armbruster) - fixed record/replay with '-rtc base' option enabled - added document with virtual hardware requirements 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 (23): 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 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: refine replay-time module 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: add BH oneshot event for block layer replay: init rtc after enabling the replay replay: document development rules util/qemu-timer: refactor deadline calculation for external timers replay: fix replay shutdown replay: rename step-related variables and functions pbonz...@redhat.com (1): replay: add missing fix for internal function MAINTAINERS | 1 Makefile.objs | 4 - accel/tcg/translator.c | 1 block/blkreplay.c | 8 + block/block-backend.c | 8 + block/io.c | 32 ++++ block/iscsi.c | 5 - block/nfs.c | 5 - block/null.c | 4 - block/nvme.c | 6 + block/qapi.c | 18 ++ block/qcow2-snapshot.c | 9 + block/qcow2.h | 2 block/rbd.c | 5 - block/vxhs.c | 5 - blockdev.c | 10 + cpus.c | 30 +++- docs/devel/replay.txt | 46 ++++++ docs/interop/qcow2.txt | 4 + docs/replay.txt | 45 ++++++ exec.c | 8 + gdbstub.c | 52 +++++++ hmp-commands-info.hx | 14 ++ hmp-commands.hx | 53 +++++++ hmp.h | 4 + include/block/snapshot.h | 1 include/qemu/timer.h | 7 - include/sysemu/replay.h | 30 ++++ migration/savevm.c | 11 ++ qapi/block-core.json | 8 + qapi/block.json | 3 qapi/misc.json | 18 -- qapi/qapi-schema.json | 1 qapi/replay.json | 121 +++++++++++++++++ qtest.c | 2 replay/Makefile.objs | 3 replay/replay-debugging.c | 326 +++++++++++++++++++++++++++++++++++++++++++++ replay/replay-events.c | 18 ++ replay/replay-internal.c | 10 + replay/replay-internal.h | 17 +- replay/replay-snapshot.c | 6 - replay/replay-time.c | 36 ++--- replay/replay.c | 56 ++++++-- stubs/Makefile.objs | 1 stubs/replay-user.c | 9 + stubs/replay.c | 10 + tests/ptimer-test-stubs.c | 2 tests/ptimer-test.c | 4 - util/qemu-timer.c | 41 +++++- vl.c | 21 ++- 50 files changed, 1007 insertions(+), 134 deletions(-) create mode 100644 docs/devel/replay.txt create mode 100644 qapi/replay.json create mode 100644 replay/replay-debugging.c create mode 100644 stubs/replay-user.c -- Pavel Dovgalyuk
