vgsp: fail4 r231 - READ-ORDER guard/witness + 32KB copy-resolution witness

r231 probe: RAWA (fresh 64 KB upload -> straight D2H readback, no kernel)
returned foreign deterministic content (0x437F0000-family ramps) where the
uploaded ints 0..16383 must be, and PROBE2 == PROBE2B on that same content.
The upload/readback round trip inside the emulation is therefore not
content-preserving - before any engine execution. Two candidate mechanisms,
both now witnessed:

- READ-ORDER: a device-src copy reading a VA whose device-dst write is
  still inside the m169 pin-guard window (the writer's bytes land only when
  the guard resolves) - the read executes first. READ-ORDER-WIT logs the
  coincidence; with sema_read_order_guard=1 (new param, default 0) the read
  is deferred through the existing bounded-hold re-decode so it runs after
  the write. Opt-in pending an A/B proving no green regression.

- RESOLUTION DIVERGENCE: the dst (H2D) resolution consults the ledger
  before svc_op_translate while the src (D2H) resolution does the opposite,
  so a stale ledger landing can make the writer and reader touch different
  pages. The sema-bigcopy proof (va->pa/aperture/provenance for BOTH sides)
  now fires from 32 KB (was 1 MB), so the probe-class copies are on record.

Build green; module param registered and visible.

(cherry picked from commit bfe5d295dd3922c0dbcd0bd1a29912a02acb58ec)

1 files changed, 56 insertions, 1 deletions

diff --git a/kernel/gsp_sema_service.c b/kernel/gsp_sema_service.c
index e57bcac..9de9038 100644
--- a/kernel/gsp_sema_service.c
+++ b/kernel/gsp_sema_service.c
@@ -3104,6 +3104,8 @@ static inline bool svc_pool_pa_identity(u64 va, u64 pa)
 
 static void svc_pool_boot_store(u64 va, u64 pa, int ap);	/* m168 */
 static bool svc_dst_guard_hold(u64 va);			/* m169b */
+static bool svc_dst_guard_pending(u64 va);		/* r231 */
+static u32 svc_read_order_log;				/* r231 budget */
 
 static int svc_pool_pa_lookup(u64 va, u64 *pa, int *ap)
 {
@@ -5241,6 +5243,24 @@ static int svc_copy(struct nv_vgpu_device *vgpu, u64 dst, u64 src, size_t total,
 				dst_orig, dst_prov);
 		return -ENOENT;
 	}
+	/*
+	 * r231 READ-ORDER guard/witness: a device-src copy whose source VA
+	 * has a PENDING device-dst write hold reads the PRE-write content -
+	 * the writer's bytes land only when the guard resolves while the
+	 * reader executes immediately (the ctaid_probe RAWA round trip is
+	 * the reproducer). Witnessed always (bounded); deferred only with
+	 * sema_read_order_guard, through the same bounded-hold re-decode.
+	 */
+	if (src_orig >= SVC_VAS_FLOOR && svc_dst_guard_pending(src_orig)) {
+		if (svc_read_order_log < 16) {
+			svc_read_order_log++;
+			pr_info("sema-svc: READ-ORDER-WIT device-src copy va=0x%llx reads while a device-dst write to the same VA is still held%s\n",
+				src_orig, gsp_sema_read_order_guard ?
+				" (deferring)" : " (guard off)");
+		}
+		if (gsp_sema_read_order_guard)
+			return -ENOENT;
+	}
 	if (!src_phys) {
 		u64 pa;
 		u32 aper;
@@ -5285,7 +5305,7 @@ static int svc_copy(struct nv_vgpu_device *vgpu, u64 dst, u64 src, size_t total,
 	 * 16 bytes on each side before the copy. The vadd data path is exactly
 	 * these copies; this separates a wrong resolution from a wrong copy.
 	 */
-	if ((total >= (1u << 20) ||
+	if ((total >= SVC_COPY_AUDIT_MIN ||
 	     (dst_virt && dst_orig >= SVC_VAS_FLOOR)) &&
 	    bigcopy_log < gsp_sema_bigcopy_log_max) {
 		u8 sb[16] = {0}, db[16] = {0};
@@ -10862,6 +10882,21 @@ bool gsp_sema_copy_dst_pin_guard = 1;
 unsigned gsp_sema_copy_dst_guard_ms = 3000;
 
 /*
+ * r231 READ-ORDER guard (default 0: witness only). A device-src copy whose
+ * source VA has a PENDING device-dst write hold (the m169 guard window is
+ * still open) reads the PRE-write content: the writer's bytes land only
+ * when the guard resolves, while the reader can execute immediately. The
+ * ctaid_probe RAWA round trip is the reproducer - upload a fresh 64 KB
+ * buffer, read it straight back with no kernel - and the readback came
+ * back as foreign deterministic content. With this on, such a read is
+ * deferred through the existing bounded-hold machinery (-ENOENT) so it
+ * re-decodes after the write resolves; with it off the condition is
+ * logged (READ-ORDER-WIT) so the next capture proves the coincidence.
+ * Opt-in until an A/B proves no green regression.
+ */
+bool gsp_sema_read_order_guard;
+
+/*
  * COPYWIT fail4: per-boot budget for the sema-bigcopy resolution witnesses
  * (svc_copy's large-copy proof: both operands' VA -> pa/aperture/provenance
  * plus the first 16 bytes each side). The shipped default kept the boot's
@@ -10911,6 +10946,26 @@ static bool svc_dst_guard_hold(u64 va)
 }
 
 /*
+ * r231 READ-ORDER guard: true while a held device-dst write for this VA
+ * page is still inside its bounded window (the writer's bytes have not
+ * landed yet). See the gsp_sema_read_order_guard declaration.
+ */
+static bool svc_dst_guard_pending(u64 va)
+{
+	u64 page = va & ~(u64)(SEMA_PAGE_SIZE - 1);
+	unsigned long now = jiffies;
+	u32 i;
+
+	for (i = 0; i < SVC_DST_GUARD_SLOTS; i++)
+		if (svc_dst_guard[i].va_page == page &&
+		    svc_dst_guard[i].first &&
+		    time_before(now, svc_dst_guard[i].first +
+				msecs_to_jiffies(gsp_sema_copy_dst_guard_ms)))
+			return true;
+	return false;
+}
+
+/*
  * CE release fallback: when a push carrying a tracking-semaphore release
  * cannot be read (and the content rescue failed), advance the channel's
  * tracking semaphore to the expected payload directly. The push's data