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[Qemu-devel] [PATCH v3 18/19] replay: describe reverse debugging in docs

From: Pavel Dovgalyuk
Subject: [Qemu-devel] [PATCH v3 18/19] replay: describe reverse debugging in docs/replay.txt
Date: Wed, 23 May 2018 09:51:24 +0300
User-agent: StGit/0.17.1-dirty

This patch updates the documentation and describes usage of the reverse
debugging in QEMU+GDB.

Signed-off-by: Pavel Dovgalyuk <address@hidden>
 docs/replay.txt |   33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/docs/replay.txt b/docs/replay.txt
index f7def53..086d3f8 100644
--- a/docs/replay.txt
+++ b/docs/replay.txt
@@ -293,6 +293,39 @@ 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.
 Replay log format

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