r453 rel_mod=1 default: route the forwarded QMD release0 to the module semaphore so the guest-visible release word is authored by the host only on the completion edge (after the retire wait and after the bounce copy-out shipped the engine's stores). With the historical engine-authored guest release the guest's stream-ordered readback could be satisfied while the host copy-out was still pending, so streams2's c2 / memcpy_roundtrip's memset phase / streams_events' chain read pre-launch content. Evidence on r453 (with the r452 FNV change-detection fix): rel_mod=0 -> 11/15 battery; rel_mod=1 -> 13/15 (victory_streams_events and memcpy_roundtrip flip green), no stalls across the full battery, victory_mail + victory_saxpy clean. The r414 stall hole is closed by the r420b per-outcome authoring; rollback is one echo to 0

1 files changed, 21 insertions, 11 deletions

diff --git a/kernel/gsp_phys_gpu.c b/kernel/gsp_phys_gpu.c
index d1aa06e..74062dd 100644
--- a/kernel/gsp_phys_gpu.c
+++ b/kernel/gsp_phys_gpu.c
@@ -734,20 +734,30 @@ int gsp_phys_rmcall_gr_qmdwin_value(void)
 EXPORT_SYMBOL_GPL(gsp_phys_rmcall_gr_qmdwin_value);
 
 /*
- * m64 forced module-semaphore release (gsp_phys_rmcall_gr_rel_mod; default 0).
- * When set, a forwarded GR launch ignores the resolved guest release page and
- * points QMD release0 at the module window-0 semaphore VA
- * (GSP_PHYS_RMCALL_VA_SEM), which the host polls at the SEM chunk's host VA.
- * Discriminant for "engine completed but the shadow release write was lost"
- * vs "engine never completed": with the module release, a completed QMD is
- * always observable. Also makes the existing module-release fallback
- * observable (it previously set no poll source, so it could never complete).
- */
-static int gsp_phys_rmcall_gr_rel_mod_param;
+ * m64 forced module-semaphore release (gsp_phys_rmcall_gr_rel_mod).
+ *
+ * DEFAULT 1 since r453. When set, a forwarded GR launch ignores the resolved
+ * guest release page and points QMD release0 at the module window-0 semaphore
+ * VA (GSP_PHYS_RMCALL_VA_SEM), which the host polls at the SEM chunk's host
+ * VA; the GUEST-VISIBLE release word is authored by the host only on the
+ * completion edge - after the retire wait and after the bounce copy-out has
+ * shipped the engine's stores to the guest pages. Evidence (r453, the FNV
+ * change-detection fix on top):
+ *   - with 0: the guest's release could be satisfied by the engine's own
+ *     early write while the host copy-out was still pending, so a client's
+ *     stream-ordered readback (streams2's c2, memcpy_roundtrip's memset
+ *     phase, streams_events' chain) read PRE-launch content: 11/15 battery.
+ *   - with 1: 13/15 battery (streams_events and memcpy_roundtrip flipped
+ *     green), no stalls across the full battery, mail+saxpy clean.
+ * The r414 stall hole (a launch that never reached the hold edge left the
+ * guest word unauthored) is closed by the r420b per-outcome authoring path;
+ * one-line rollback: echo 0 > .../parameters/gsp_phys_rmcall_gr_rel_mod.
+ */
+static int gsp_phys_rmcall_gr_rel_mod_param = 1;
 module_param_named(gsp_phys_rmcall_gr_rel_mod,
 		   gsp_phys_rmcall_gr_rel_mod_param, int, 0644);
 MODULE_PARM_DESC(gsp_phys_rmcall_gr_rel_mod,
-		 "Force the forwarded GR QMD release0 to the module semaphore (default 0; 1 = m64 engine-completion discriminant)");
+		 "Force the forwarded GR QMD release0 to the module semaphore; guest release authored only on the post-copy-out completion edge (default 1 since r453; 0 = historical engine-authored guest release)");
 
 int gsp_phys_rmcall_gr_rel_mod_value(void)
 {