diff --git a/tests/unit/test_pv_ss_b64.cu b/tests/unit/test_pv_ss_b64.cu
new file mode 100644
index 00000000..dd687731
--- /dev/null
+++ b/tests/unit/test_pv_ss_b64.cu
@@ -0,0 +1,88 @@
+/**
+ * Minimal test: PV SS MMA with B=(64,16) BLOCK_MN=64.
+ * Tests if BLOCK_MN=64 is valid for the UMMA B descriptor.
+ */
+
+#include <cuda_runtime.h>
+#include <cstdio>
+#include <cstring>
+
+#include "dsv4/kernels/attention/fmha_common.cuh"
+#include "dsv4/kernels/attention/fmha_umma_desc.cuh"
+
+using namespace dsv4::kernels::attention;
+
+static bf16_t f32_to_bf16_host(float f) { uint32_t u; memcpy(&u,&f,4); return (uint16_t)(u>>16); }
+
+constexpr int HD = 64, BLOCK_MN = 128;
+
+__global__ void __launch_bounds__(128)
+test_pv_ss_b64()
+{
+    const int tid = threadIdx.x, wid = tid / 32, lane = tid % 32;
+
+    extern __shared__ char sbuf[];
+    uint32_t* sTmemBase = (uint32_t*)sbuf;
+    bf16_t* sP = (bf16_t*)(((uintptr_t)(sbuf + 4) + 15) & ~(uintptr_t)15);
+    bf16_t* sV = (bf16_t*)(((uintptr_t)(sP + 128 * 16) + 127) & ~(uintptr_t)127);
+
+    // Fill P: (128, 16) canonical, row 0 = all 1.0
+    constexpr int CORES_MN = 16, CORES_K = 2;
+    for (int i = tid; i < 128 * 16; i += 128) sP[i] = 0;
+    __syncthreads();
+    for (int c = tid; c < 16; c += 128) {
+        int ck = c / 8, lc = c % 8;
+        sP[ck * CORES_MN * 64 + 0 * 64 + 0 * 8 + lc] = f32_to_bf16(1.0f);
+    }
+    __syncthreads();
+
+    // Fill V: (64, 16) canonical with BLOCK_MN=64
+    // CORES_MN=8, CORES_K=2
+    // B[d, r]: g_mn=d/8, g_k=r/8, llr=d%8, lc=r%8
+    for (int i = tid; i < 64 * 16; i += 128) sV[i] = 0;
+    __syncthreads();
+    for (int d = tid; d < HD; d += 128) {
+        for (int r = 0; r < 16; r++) {
+            int g_mn = d / 8, g_k = r / 8;
+            int llr = d % 8, lc = r % 8;
+            sV[g_k * 8 * 64 + g_mn * 64 + llr * 8 + lc] = f32_to_bf16(2.0f);
+        }
+    }
+    __syncthreads();
+
+    if (wid == 1) tmem_alloc(__cvta_generic_to_shared(sTmemBase), 128);
+    __syncthreads();
+    uint32_t tb = *sTmemBase;
+
+    // PV SS MMA: A=(128,16) BLOCK_MN=128, B=(64,16) BLOCK_MN=64
+    // C = A × B^T = (128, 64). idesc: MMA_M=8, MMA_N=4
+    uint64_t dp = make_umma_desc_kmajor_none(__cvta_generic_to_shared(sP), BLOCK_MN);
+    uint64_t dv = make_umma_desc_kmajor_none(__cvta_generic_to_shared(sV), HD);
+    uint32_t idesc = make_idesc(BLOCK_MN, HD);
+    if (tid == 0) umma_ss_f16(tb, dp, dv, idesc, false);
+    asm volatile("tcgen05.fence::after_thread_sync;" ::: "memory");
+    __syncthreads();
+
+    // Read first 8 cols of C
+    if (wid == 0) {
+        float tmp[8];
+        asm volatile("tcgen05.ld.sync.aligned.32x32b.x8.b32 {%0,%1,%2,%3,%4,%5,%6,%7},[%8];"
+            : "=f"(tmp[0]),"=f"(tmp[1]),"=f"(tmp[2]),"=f"(tmp[3]),
+              "=f"(tmp[4]),"=f"(tmp[5]),"=f"(tmp[6]),"=f"(tmp[7])
+            : "r"(tb));
+        asm volatile("tcgen05.wait::ld.sync.aligned;");
+        if (lane == 0) { printf("C[0,0..7]: "); for(int c=0;c<8;c++) printf("%.1f ", tmp[c]); printf("\n"); }
+    }
+
+    if (wid == 0) tmem_dealloc(tb, 128);
+}
+
+int main() {
+    printf("=== PV SS MMA B=(64,16) BLOCK_MN=64 ===\n");
+    int smem = (4+16 + 128*16*2 + 64*16*2 + 256 + 127) & ~127;
+    printf("SMEM: %d bytes\n", smem);
+    test_pv_ss_b64<<<1, 128, smem>>>();
+    cudaError_t err = cudaDeviceSynchronize();
+    if (err != cudaSuccess) { printf("CUDA ERROR: %s\n", cudaGetErrorString(err)); return 1; }
+    return 0;
+}