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Sun, 23 Aug 2026 05:31:59 -0700 (PDT) MIME-Version: 1.0 From: Dalbir Singh Date: Sun, 23 Aug 2026 13:31:48 +0100 X-Gm-Features: AcwNN1WXdw4g6uzGq7RF2nx1wQf-hL28TVIUkDaYtHTpue22eUlpnKTb8qefsPY Message-ID: Subject: [PATCH] plugins/lockstep: document plugin logging requirement To: qemu-devel@nongnu.org Cc: alex.bennee@linaro.org, pierrick.bouvier@oss.qualcomm.com, erdnaxe@crans.org Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists1p.gnu.org; Received-SPF: pass client-ip=2607:f8b0:4864:20::231; envelope-from=dalbirrana838@gmail.com; helo=mail-oi1-x231.google.com X-Spam_score_int: -17 X-Spam_score: -1.8 X-Spam_bar: - X-Spam_report: (-1.8 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_ENVFROM_END_DIGIT=0.25, FREEMAIL_FROM=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-Mailman-Approved-At: Sun, 23 Aug 2026 08:41:55 -0400 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @gmail.com) X-ZM-MESSAGEID: 1787488965425158500 Content-Type: text/plain; charset="utf-8" plugins/lockstep: document plugin logging requirement qemu_plugin_outs() output from this plugin =E2=80=94 including the "No divergence :-)" summary and any divergence reports =E2=80=94 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 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 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) *