[PATCH] plugins/lockstep: document plugin logging requirement

plugins/lockstep: document plugin logging requirement

qemu_plugin_outs() output from this plugin — including the
"No divergence :-)" summary and any divergence reports — is routed
through qemu_log_mask(CPU_LOG_PLUGIN, ...), which is silent unless
QEMU's own logging is enabled. A user running the plugin exactly as
shown in this file's own usage will see no output at all and can
reasonably conclude the plugin is doing nothing.

Document that -d plugin -D <logfile> is required to see any output.

Found via manual, reproducible experimentation (deterministic guest
binary, controlled single-instruction divergence, TB-address and
instruction-count cross-checked against objdump) with AI (Claude)
assistance in the investigation; the missing-output behavior and the
fix were independently verified by hand before this patch was written.

Signed-off-by: Dalbir <[email protected]>

diff --git a/contrib/plugins/lockstep.c b/contrib/plugins/lockstep.c
index xxxxxxx..xxxxxxx 100644
--- a/contrib/plugins/lockstep.c
+++ b/contrib/plugins/lockstep.c
@@ -7,6 +7,12 @@
  * introduced a subtle and hard to find bug.
  *
  * Caveats:
+ *   - qemu_plugin_outs() output (including "No divergence :-)" and any
+ *     divergence reports) is routed through the plugin log mask, so it is
+ *     silent unless QEMU's own logging is enabled, e.g.:
+ *         -d plugin -D /path/to/logfile
+ *     Without -d plugin, the plugin runs and compares correctly but
+ *     produces no visible output at all.
  *   - single-threaded linux-user apps only with non-deterministic syscalls
  *   - no MTTCG enabled system emulation (icount may help)
  *