From e5d1b3d6ab61b85b945400fcbd3637052feb2ace Mon Sep 17 00:00:00 2001
From: Arthur Hanson Rasmusson <arthur@vgpu.io>
Date: Wed, 30 Sep 2026 05:23:28 -0400
Subject: sema-svc: sema_copy_watch (size-keyed src/dst byte audit for the CE
 data path)

---
 kernel/gsp_sema_service.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

(limited to 'kernel/gsp_sema_service.c')

diff --git a/kernel/gsp_sema_service.c b/kernel/gsp_sema_service.c
index 56285bf..4d3f6ef 100644
--- a/kernel/gsp_sema_service.c
+++ b/kernel/gsp_sema_service.c
@@ -343,6 +343,7 @@ struct sema_svc {
 	u64			memsets_fail;
 	u64			ring_ops;		/* op runs from ring-decoded pushes */
 	u64			lda_log;		/* LAUNCH_DMA inventory budget */
+	u64			copy_watch_log;		/* sema_copy_watch log budget */
 	u64			entry_log;		/* per-push summary budget */
 	u64			ops_denied;		/* work refused: implausible stream */
 	u64			false_trans;		/* resolvable VA mapped to non-push */
@@ -2019,6 +2020,41 @@ static int svc_copy(struct nv_vgpu_device *vgpu, u64 dst, u64 src, size_t total,
 		src_aperture = aper;
 	}
 
+	/*
+	 * Data-path watch: for copies of exactly sema_copy_watch bytes, log
+	 * the first 8 source and destination bytes so a broken link in the
+	 * chain (wrong source page, wrong destination, silent wipe) is
+	 * visible directly in the log.
+	 */
+	if (gsp_sema_copy_watch && total == gsp_sema_copy_watch) {
+		u8 sb[8] = {0}, db[8] = {0};
+
+		if (src_aperture == CE_APERTURE_VID) {
+			u8 *base = vgpu->bars[1].vaddr;
+
+			if (base && src + 8 <= vgpu->bars[1].size)
+				memcpy(sb, base + src, 8);
+		} else {
+			svc_read(vgpu, src, sb, 8);
+		}
+		if (dst_aperture == CE_APERTURE_VID) {
+			u8 *base = vgpu->bars[1].vaddr;
+
+			if (base && dst + 8 <= vgpu->bars[1].size)
+				memcpy(db, base + dst, 8);
+		} else {
+			svc_read(vgpu, dst, db, 8);
+		}
+		if (svc.copy_watch_log < 64) {
+			svc.copy_watch_log++;
+			pr_info("sema-copy-watch: total=%zu dst=0x%llx dap=%u src=0x%llx sap=%u src8=%02x %02x %02x %02x %02x %02x %02x %02x dst8_before=%02x %02x %02x %02x %02x %02x %02x %02x\n",
+				total, dst, dst_aperture, src, src_aperture,
+				sb[0], sb[1], sb[2], sb[3], sb[4], sb[5],
+				sb[6], sb[7], db[0], db[1], db[2], db[3],
+				db[4], db[5], db[6], db[7]);
+		}
+	}
+
 	for (o = 0; o < total; ) {
 		size_t chunk = total - o;
 		u32 i;
@@ -3685,6 +3721,9 @@ bool gsp_sema_skip_ops;
 /* Executed-op log cap (see header). */
 unsigned gsp_sema_ops_log = 8;
 
+/* Data-path watch: copy size whose first bytes are logged (0 = off). */
+unsigned gsp_sema_copy_watch;
+
 static unsigned long svc_gr_ring_tried;	/* ranks that got a diagnostic line */
 
 static void svc_gr_ring_autopin(struct nv_vgpu_device *vgpu,
-- 
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