Pavel Dovgalyuk <pavel.dovgal...@ispras.ru> writes:
> From: Pavel Dovgalyuk <pavel.dovga...@ispras.ru>
>
> This patch updates the documentation and describes usage of the reverse
> debugging in QEMU+GDB.
>
> Signed-off-by: Pavel Dovgalyuk <pavel.dovgal...@ispras.ru>
> ---
> docs/replay.txt | 33 +++++++++++++++++++++++++++++++++
> 1 file changed, 33 insertions(+)
Minor aside - it is probably worth having a separate patch to convert
this file into an .rst and put it in the docs/system folder so we can
properly incorporate it into the user documentation.
>
> diff --git a/docs/replay.txt b/docs/replay.txt
> index 70c27edb36..18d6169f3b 100644
> --- a/docs/replay.txt
> +++ b/docs/replay.txt
> @@ -294,6 +294,39 @@ for recording and replaying must contain identical
> number of ports in record
> and replay modes, but their backends may differ.
> E.g., '-serial stdio' in record mode, and '-serial null' in replay mode.
>
> +Reverse debugging
> +-----------------
> +
> +Reverse debugging allows "executing" the program in reverse direction.
> +GDB remote protocol supports "reverse step" and "reverse continue"
> +commands. The first one steps single instruction backwards in time,
> +and the second one finds the last breakpoint in the past.
> +
> +Recorded executions may be used to enable reverse debugging. QEMU can't
> +execute the code in backwards direction, but can load a snapshot and
> +replay forward to find the desired position or breakpoint.
> +
> +The following GDB commands are supported:
> + - reverse-stepi (or rsi) - step one instruction backwards
> + - reverse-continue (or rc) - find last breakpoint in the past
> +
> +Reverse step loads the nearest snapshot and replays the execution until
> +the required instruction is met.
> +
> +Reverse continue may include several passes of examining the execution
> +between the snapshots. Each of the passes include the following steps:
> + 1. loading the snapshot
> + 2. replaying to examine the breakpoints
> + 3. if breakpoint or watchpoint was met
> + - loading the snaphot again
> + - replaying to the required breakpoint
> + 4. else
> + - proceeding to the p.1 with the earlier snapshot
> +
> +Therefore usage of the reverse debugging requires at least one snapshot
> +created in advance. See the "Snapshotting" section to learn about running
> +record/replay and creating the snapshot in these modes.
I would explicitly state you need:
- a block device for storing VM snapshots (independent of storage
devices you may have)
- to specify the starting rrsnapshot in both the record and replay runs
Otherwise:
Reviewed-by: Alex Bennée <alex.ben...@linaro.org>
--
Alex Bennée
