vgsp-gr: m156 fail-closed release gate - backend half (witness + deferred release0)

The engine's QMD/channel completion ack is a front-end retirement, not an
execution witness. r155 live evidence: streams2's launch logged
qmd_rel=6/ch_rel=0xc0ffee02 while the retirement wait saw '0/205 pages
changed' on the initial attempt and all three outcome retries; the backend
then returned success and the client read all-zero output (STREAMS
bad=32768). The r152 matmul/smiley boots show the same signature.

For a data-bearing launch (resolved operand pages + working bounce window):

- new param gsp_phys_rmcall_gr_failclosed (default 1) forces release0 to
  the module semaphore (the m64 rel-mod shape), so the engine's ack cannot
  reach the guest's release word before an executor is witnessed;
- when the retirement wait still saw zero writes after the bounded
  retries, the backend returns -EIO instead of success and logs
  'FAIL-CLOSED: ... success withheld', so the sema service can withhold
  the guest release and fail loudly (companion commit).

Set gsp_phys_rmcall_gr_failclosed=0 to restore the historical fail-open
behavior. Build: make -j8 (kernel), green.

1 files changed, 55 insertions, 0 deletions

diff --git a/kernel/gsp_phys_booter.c b/kernel/gsp_phys_booter.c
index 9eb1179..0c27853 100644
--- a/kernel/gsp_phys_booter.c
+++ b/kernel/gsp_phys_booter.c
@@ -16976,6 +16976,16 @@ int gsp_phys_backend_guest_gr_submit(const struct gsp_phys_gr_submit *req,
 	char qmdwin_note[32] = "";
 	bool rearmed = false;
 	u32 attempts = 0;
+	/*
+	 * m156 fail-closed gate: for a data-bearing launch (resolved operand
+	 * pages plus the working bounce window) release0 is routed to the
+	 * module semaphore so the engine's completion ack cannot reach the
+	 * guest's release word, and success is withheld when the retirement
+	 * wait still observed zero engine writes. See the param comment in
+	 * gsp_phys_gpu.c.
+	 */
+	bool fail_closed = false;
+	u32 writes_seen = 0;
 	int ret = 0;
 
 	if (out)
@@ -17050,6 +17060,15 @@ int gsp_phys_backend_guest_gr_submit(const struct gsp_phys_gr_submit *req,
 		qmd[4] |= 1u << 12;
 
 	/*
+	 * m156: a data-bearing launch whose resolved pages can actually be
+	 * witnessed (the bounce window is active) is subject to the
+	 * fail-closed release gate.
+	 */
+	if (gsp_phys_rmcall_gr_failclosed_value() && req->n_data &&
+	    boot_stage.rmcall_gr_bounce.pages)
+		fail_closed = true;
+
+	/*
 	 * QMD release0: when the launch has a one-word absolute release and
 	 * the caller resolved the guest release page, author it there through
 	 * a fresh shadow line so the guest observes its own QMD release; else
@@ -17062,6 +17081,21 @@ int gsp_phys_backend_guest_gr_submit(const struct gsp_phys_gr_submit *req,
 		u32 one_word = (qmd[24] >> 31) & 1;
 		bool rel_mod = gsp_phys_rmcall_gr_rel_mod_value() != 0;
 
+		/*
+		 * m156: force the module-semaphore release for a data-bearing
+		 * launch under the fail-closed gate. The engine authors the
+		 * ack into the module sem (host-visible only); the guest
+		 * release word is authored later by the sema service, and
+		 * only when the backend witnessed engine writes. Without
+		 * this the engine's retirement ack lands in the guest's
+		 * release word even when no executor produced data, and the
+		 * client reads stale/zero results as success (r155 live:
+		 * streams2 bad=32768 with "0/205 pages changed" on every
+		 * attempt).
+		 */
+		if (fail_closed)
+			rel_mod = true;
+
 		if (!rel_mod && req->rel_hpa && one_word && !red_en &&
 		    n_shadow < GSP_PHYS_RMCALL_GR_SHADOW_PAGES) {
 			u64 sh_va = 0;
@@ -18388,6 +18422,7 @@ gr_page_unmapped:
 	if (req->n_data && boot_stage.rmcall_gr_bounce.pages) {
 		u32 wrote = gsp_phys_gr_output_retire_wait(req, bounce_csum_in);
 
+		writes_seen = wrote;
 		if (gsp_phys_rmcall_gr_nowrite_retry_value()) {
 			while (attempts < GSP_PHYS_RMCALL_GR_ATTEMPTS) {
 				if (wrote > 0)
@@ -18447,6 +18482,7 @@ gr_page_unmapped:
 				}
 				wrote = gsp_phys_gr_output_retire_wait(
 					req, bounce_csum_in);
+				writes_seen = wrote;
 				pr_info("vgsp-gr: outcome retry %u done: engine wrote %u page(s)\n",
 					attempts, wrote);
 			}
@@ -18536,6 +18572,25 @@ gr_page_unmapped:
 			wrote, changed, chg[0], chg[1], chg[2], chg[3], chg[4],
 			chg[5], chg[6], chg[7]);
 	}
+	/*
+	 * m156 fail-closed: a data-bearing launch that acked (qmd_rel and
+	 * ch_rel observed) but whose retirement wait saw no engine write on
+	 * any attempt must not be reported as success. The completion ack is
+	 * a front-end retirement, not an execution witness (r155 live:
+	 * streams2 read all-zero output after exactly this signature, while
+	 * this function returned 0 and the client's release advanced). Return
+	 * an error so the sema service withholds the guest release and
+	 * delivers a channel error; release0 was routed to the module
+	 * semaphore above, so the engine's ack never reached the guest word.
+	 */
+	if (fail_closed && writes_seen == 0) {
+		pr_err("vgsp-gr: FAIL-CLOSED: data-bearing launch acked (qmd_rel=0x%08x ch_rel=0x%08x) but the engine wrote 0 of %u resolved pages after %u attempt(s) (elapsed_ms=%u); success withheld (rc=-EIO)\n",
+		       w0, w1, req->n_data, attempts + 1u,
+		       jiffies_to_msecs(jiffies - start));
+		poll = w1;
+		ret = -EIO;
+		goto out;
+	}
 	poll = w1;
 	if (bar0)
 		pr_info("vgsp-gr: SKED readback (ok): HWW_ESR=0x%08x INFO2=0x%08x CWD_T0=0x%08x CWD_T1=0x%08x CWD_T2=0x%08x\n",