vgsp: client-session boundary fixes - WD ring scan gate + forced session flush

Two changes from the r839/r840 multi-client forensics (session 147):

1. The guest-RAM content scan for a GR ring must not run for the kernel
   RC-watchdog channel (h=0x31415900). Its ring is a plain kernel allocation
   always resolvable through the driver-published VAS walk; on r840 the walk
   lost a registration race, the scan latched a foreign garbage page, and
   the bad latch poisoned the driver's DMA test ('Failed to initialize DMA',
   Xorg died). Gated, the walk retries per pass; r841 Xorg#1 passes DMA
   (DMAFAIL=0) where r840 failed.

2. sema_force_flush module parameter + gsp_sema_service_sess_flush_force():
   execute the ISSUE-0002 client-session flush even with live channels. The
   channel-close trigger never fires on real boots (driver-global channels
   stay valid), so later client sessions inherited stale discovery state.
   Verified on r841: flush executes mid-boot, Xorg survives it and
   re-registers its channels.

Remaining wall (addendum 9): GR client channels' pushbuffer reads return
all-zero through the trusted translation (va=0x4000000-class, src=4), so
first entries never complete and GL clients never render.

1 files changed, 48 insertions, 3 deletions

diff --git a/kernel/gsp_sema_service.c b/kernel/gsp_sema_service.c
index f6ad3b2..9c72b4b 100644
--- a/kernel/gsp_sema_service.c
+++ b/kernel/gsp_sema_service.c
@@ -678,6 +678,9 @@ static u32 svc_ring_n;
  */
 static u8 svc_sess_flush;
 static u8 svc_guest_reset_req;	/* m177: guest RM reload; wiped on svc thread */
+static u8 svc_sess_flush_forced;	/* execute the next flush past the live-channel skip
+					 * (ISSUE-0002 client-session boundary aid; see
+					 * gsp_sema_service_sess_flush_force) */
 static u8 svc_sess_active;	/* a channel of a session has registered (until flush) */
 static u32 svc_sess_gen;	/* m176: bumped by every executed session flush;
 				 * stamps channels/scrubbers so per-session learned
@@ -13415,6 +13418,7 @@ static void svc_session_flush(void)
 	 */
 	{
 		bool live = false;
+		bool forced = READ_ONCE(svc_sess_flush_forced);
 
 		spin_lock_irqsave(&svc.chan_lock, flags);
 		for (i = 0; i < SVC_MAX_CHANNELS; i++)
@@ -13423,10 +13427,15 @@ static void svc_session_flush(void)
 				break;
 			}
 		spin_unlock_irqrestore(&svc.chan_lock, flags);
-		if (live) {
+		if (live && !forced) {
 			pr_info("sema-svc: session flush skipped: live channel(s) present (deferred flush raced a new session registration)\n");
 			return;
 		}
+		if (forced) {
+			svc_sess_flush_forced = 0;
+			pr_info("sema-svc: session flush (forced) executing with live=%d channel(s)\n",
+				live);
+		}
 	}
 
 	svc_gr_diag_reset();		/* m150: per-session diagnostic budgets */
@@ -17532,8 +17541,24 @@ static void svc_gr_ring_autopin(struct nv_vgpu_device *vgpu,
 	 * entry and pushbuffer reads resolve. Sliced so the service thread
 	 * stays responsive; gives up per channel after
 	 * SVC_GR_RAM_PASS_MAX slices.
-	 */
-	if (ch->gr_ram_passes < SVC_GR_RAM_PASS_MAX &&
+	 *
+	 * SESSION 147 (r840 evidence): the scan must NOT run for the kernel
+	 * RC-watchdog channel (h=0x31415900). Its ring is a plain kernel
+	 * allocation whose VA is always resolvable through the driver-
+	 * published VAS walk - the walk is the correct, deterministic
+	 * source. On r840 the walk lost a registration race, the scan ran
+	 * first and latched a FOREIGN page (pa=0x101cc3000, garbage
+	 * entries: raw=0xffff88f10403e000), and the bad latch poisoned
+	 * every later entry/push read: the driver's DMA test never saw its
+	 * push complete and Xorg died with "Failed to initialize DMA"
+	 * (the same boot passed on r835/836/838 where the trusted walk
+	 * won the race). Gating the scan keeps the per-pass walk retry
+	 * alive for the watchdog; the walk landing late is indistinguishable
+	 * from the r838 flow, and never landing is no worse than a poisoned
+	 * latch.
+	 */
+	if (ch->h_object != 0x31415900u &&
+	    ch->gr_ram_passes < SVC_GR_RAM_PASS_MAX &&
 	    !test_bit(ch->gr_index, &svc_gr_ring_done)) {
 		u64 hit_pa = 0;
 
@@ -30684,6 +30709,26 @@ static void svc_stall_watch_check(void)
 	svc_force_consume = 4;
 }
 
+/*
+ * ISSUE-0002 client-session boundary aid: force a session flush that
+ * executes even while channels are live. The channel-close trigger only
+ * fires when the table empties, which never happens on a guest where the
+ * driver-global kernel channels (RC watchdog 0x31415900, the tiny notifier
+ * channel) stay valid; later client sessions then inherit the previous
+ * session's map/pin/pool discovery state and wedge (false translations ->
+ * failed rescues -> driver DMA-init timeout). Raised through the
+ * sema_force_flush module parameter so the boundary can be tested before
+ * wiring automatic detection.
+ */
+void gsp_sema_service_sess_flush_force(void)
+{
+	WRITE_ONCE(svc_sess_flush_forced, 1);
+	WRITE_ONCE(svc_sess_flush, 1);
+	if (READ_ONCE(svc.task))
+		wake_up_process(svc.task);
+	pr_info("sema-svc: forced session flush requested\n");
+}
+
 static int svc_thread(void *arg)
 {
 	struct nv_vgpu_device *vgpus[SVC_MAX_GUESTS];