From a6cee384160b3210911838f79a8082d541da00f5 Mon Sep 17 00:00:00 2001
From: Arthur Hanson Rasmusson <arthur@vgpu.io>
Date: Tue, 29 Sep 2026 16:20:42 -0400
Subject: sema-svc: never consume a CE entry whose release was not delivered

The fake-release fallback acknowledged unreadable CE pushes even when the
tracking-semaphore pool page had no translation yet, so the GPFIFO entry was
consumed without writing the semaphore the client waits on (observed: client
stuck in UVM_GPU_REGISTER while CE channel 2's release never landed, its
push-buffer pin arriving ~2 s later).

- svc_fake_release now reports failure when the pool page cannot be
  translated/read/written instead of pretending success.
- CE channels with the fallback enabled leave the entry pending (no
  multi-second RAM rescue) until the pool pin lands; the skip path only
  consumes an entry when the release was actually delivered.
---
 kernel/gsp_sema_service.c | 69 ++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 59 insertions(+), 10 deletions(-)

(limited to 'kernel/gsp_sema_service.c')

diff --git a/kernel/gsp_sema_service.c b/kernel/gsp_sema_service.c
index 5bff6ca..21f0de0 100644
--- a/kernel/gsp_sema_service.c
+++ b/kernel/gsp_sema_service.c
@@ -3990,13 +3990,24 @@ static bool svc_fake_release(struct nv_vgpu_device *vgpu, struct svc_channel *ch
 	pool_va = ch->last_rel_va ? ch->last_rel_va :
 		  0x121000000ULL + 4ULL * ch->ce_index;
 
-	if (!svc_lookup_trusted(vgpu, pool_va, &spa, &sap) &&
-	    !svc_read_ap(vgpu, spa, sap, &scur, sizeof(scur)) &&
-	    scur < ch->expected_sem_payload) {
+	/*
+	 * The release can only be delivered when the pool page translates.
+	 * A missing translation is transient (the client's pool page is pinned
+	 * a moment after the channel appears), so report failure and let the
+	 * caller keep the GPFIFO entry pending: consuming it without writing
+	 * the semaphore leaves the client waiting forever.
+	 */
+	if (svc_lookup_trusted(vgpu, pool_va, &spa, &sap))
+		return false;
+	if (svc_read_ap(vgpu, spa, sap, &scur, sizeof(scur)))
+		return false;
+
+	if (scur < ch->expected_sem_payload) {
 		u32 nv = ch->expected_sem_payload;
 
-		if (!svc_write_ap(vgpu, spa, sap, &nv, sizeof(nv)))
-			svc.releases_applied++;
+		if (svc_write_ap(vgpu, spa, sap, &nv, sizeof(nv)))
+			return false;
+		svc.releases_applied++;
 	}
 	svc.releases_faked++;
 	ch->expected_sem_payload++;
@@ -4212,8 +4223,8 @@ static void svc_pump_channels(struct nv_vgpu_device *vgpu)
 						 * progress waits expire. Acknowledge
 						 * the release and move on.
 						 */
-						if (gsp_sema_fake_rel &&
-						    svc_fake_release(vgpu, ch)) {
+if (gsp_sema_fake_rel &&
+					    svc_fake_release(vgpu, ch)) {
 							ch->last_get = (idx + 1) %
 								       ch->gpfifo_entries;
 							svc.gpfifo_skipped++;
@@ -4221,6 +4232,29 @@ static void svc_pump_channels(struct nv_vgpu_device *vgpu)
 							kfree(pb);
 							continue;
 						}
+						if (gsp_sema_fake_rel &&
+						    ch->engine_type) {
+							/*
+							 * CE channel and the
+							 * pool page is not
+							 * translated yet: the
+							 * pin lands a moment
+							 * later, so keep the
+							 * entry pending
+							 * instead of burning a
+							 * multi-second RAM
+							 * rescue. Consuming it
+							 * now would drop the
+							 * release the client
+							 * waits on.
+							 */
+							if (!ch->defer_since)
+								ch->defer_since =
+									jiffies;
+							svc.gpfifo_nomap++;
+							kfree(pb);
+							break;
+						}
 						if (svc.false_trans && svc.false_trans < 8)
 							pr_info("sema-svc: false translation for push va=0x%llx (h=0x%x len=%u) - rescuing\n",
 								va, ch->h_object, len);
@@ -4305,11 +4339,26 @@ static void svc_pump_channels(struct nv_vgpu_device *vgpu)
 							pr_info("sema-chan: skipping stale entry h=0x%x va=0x%llx len=%u after %u failed rescues\n",
 								ch->h_object, va, len,
 								ch->rescue_fail);
+							if (gsp_sema_fake_rel &&
+							    !svc_fake_release(vgpu, ch)) {
+								/*
+								 * The release could not be
+								 * delivered (pool page not
+								 * translated yet): keep the
+								 * entry pending so a pin that
+								 * lands shortly still applies
+								 * it instead of silently
+								 * dropping the client's wait.
+								 */
+								if (!ch->defer_since)
+									ch->defer_since =
+										jiffies;
+								svc.gpfifo_nomap++;
+								kfree(pb);
+								break;
+							}
 							ch->rescue_fail = 0;
 							ch->defer_since = 0;
-							if (gsp_sema_fake_rel)
-								svc_fake_release(vgpu,
-										 ch);
 							ch->last_get = (idx + 1) %
 								       ch->gpfifo_entries;
 							svc.gpfifo_skipped++;
-- 
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