phase 5: naive multi-head attention

- Batched GEMM via cublasGemmStridedBatchedEx
- Causal mask CUDA kernel (F32 + BF16)
- Element-wise scale CUDA kernel (F32 + BF16)
- attention() composing: batched_matmul + scale + causal_mask + softmax
- Fixed to_device/contiguous infinite recursion (GPU contiguous via CPU round-trip)
- 5 attention tests passing (max_err < 3e-7 F32)
- Total: 61 tests passing across all crates

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/xserv-kernels/src/activation.rs

@@ -6,6 +6,8 @@ unsafe extern "C" {
     fn launch_gelu_bf16(x: *const c_void, out: *mut c_void, n: i32, stream: *mut c_void);
     fn launch_silu_f32(x: *const c_void, out: *mut c_void, n: i32, stream: *mut c_void);
     fn launch_silu_bf16(x: *const c_void, out: *mut c_void, n: i32, stream: *mut c_void);
+    fn launch_scale_f32(x: *const c_void, out: *mut c_void, scale: f32, n: i32, stream: *mut c_void);
+    fn launch_scale_bf16(x: *const c_void, out: *mut c_void, scale: f32, n: i32, stream: *mut c_void);
 }
 
 pub fn gelu(x: &Tensor) -> Tensor {
@@ -39,3 +41,19 @@ pub fn silu(x: &Tensor) -> Tensor {
     xserv_cuda::device::synchronize().unwrap();
     out
 }
+
+pub fn scale(x: &Tensor, scale_val: f32) -> Tensor {
+    assert!(x.is_contiguous());
+    assert!(matches!(x.device(), Device::Cuda(_)));
+    let out = Tensor::zeros(x.shape(), x.dtype(), x.device());
+    let n = x.numel() as i32;
+    unsafe {
+        match x.dtype() {
+            DType::F32 => launch_scale_f32(x.data_ptr() as _, out.data_ptr() as *mut c_void, scale_val, n, std::ptr::null_mut()),
+            DType::BF16 => launch_scale_bf16(x.data_ptr() as _, out.data_ptr() as *mut c_void, scale_val, n, std::ptr::null_mut()),
+            _ => panic!("unsupported dtype for scale"),
+        }
+    }
+    xserv_cuda::device::synchronize().unwrap();
+    out
+}

crates/xserv-kernels/src/attention.rs

@@ -0,0 +1,77 @@
+use std::ffi::c_void;
+use xserv_tensor::{DType, Tensor};
+
+use crate::activation::scale;
+use crate::gemm::batched_matmul;
+use crate::softmax::softmax;
+
+unsafe extern "C" {
+    fn launch_causal_mask_f32(scores: *mut c_void, batch: i32, rows: i32, cols: i32,
+                              offset: i32, stream: *mut c_void);
+    fn launch_causal_mask_bf16(scores: *mut c_void, batch: i32, rows: i32, cols: i32,
+                               offset: i32, stream: *mut c_void);
+}
+
+fn apply_causal_mask(scores: &Tensor, offset: usize) {
+    let ndim = scores.ndim();
+    let rows = scores.shape()[ndim - 2];
+    let cols = scores.shape()[ndim - 1];
+    let batch: usize = scores.shape()[..ndim - 2].iter().product();
+
+    unsafe {
+        match scores.dtype() {
+            DType::F32 => launch_causal_mask_f32(
+                scores.data_ptr() as *mut c_void,
+                batch as i32, rows as i32, cols as i32, offset as i32,
+                std::ptr::null_mut(),
+            ),
+            DType::BF16 => launch_causal_mask_bf16(
+                scores.data_ptr() as *mut c_void,
+                batch as i32, rows as i32, cols as i32, offset as i32,
+                std::ptr::null_mut(),
+            ),
+            _ => panic!("unsupported dtype for causal mask"),
+        }
+    }
+    xserv_cuda::device::synchronize().unwrap();
+}
+
+/// Multi-head attention (naive, materializes S×S score matrix).
+///
+/// q, k, v: [batch, num_heads, seq_len, head_dim] — contiguous, on GPU
+/// Returns: [batch, num_heads, seq_len, head_dim]
+pub fn attention(q: &Tensor, k: &Tensor, v: &Tensor, causal: bool) -> Tensor {
+    assert_eq!(q.ndim(), 4);
+    assert_eq!(k.ndim(), 4);
+    assert_eq!(v.ndim(), 4);
+    assert!(q.is_contiguous() && k.is_contiguous() && v.is_contiguous());
+
+    let batch = q.shape()[0];
+    let num_heads = q.shape()[1];
+    let q_len = q.shape()[2];
+    let head_dim = q.shape()[3];
+    let kv_len = k.shape()[2];
+
+    assert_eq!(k.shape(), &[batch, num_heads, kv_len, head_dim]);
+    assert_eq!(v.shape(), &[batch, num_heads, kv_len, head_dim]);
+
+    // scores = Q @ K^T → [B, H, q_len, kv_len]
+    let k_t = k.transpose(2, 3).contiguous();
+    let scores = batched_matmul(q, &k_t);
+
+    // Scale by 1/sqrt(head_dim)
+    let scale_factor = 1.0 / (head_dim as f32).sqrt();
+    let scaled_scores = scale(&scores, scale_factor);
+
+    // Causal mask
+    if causal {
+        let offset = kv_len - q_len;
+        apply_causal_mask(&scaled_scores, offset);
+    }
+
+    // Softmax
+    let weights = softmax(&scaled_scores);
+
+    // output = weights @ V → [B, H, q_len, head_dim]
+    batched_matmul(&weights, v)
+}

crates/xserv-kernels/src/gemm.rs

@@ -46,6 +46,19 @@ unsafe extern "C" {
         compute_type: i32,
         algo: i32,
     ) -> i32;
+    fn cublasGemmStridedBatchedEx(
+        handle: CublasHandle,
+        transa: i32, transb: i32,
+        m: i32, n: i32, k: i32,
+        alpha: *const c_void,
+        a: *const c_void, a_type: i32, lda: i32, stride_a: i64,
+        b: *const c_void, b_type: i32, ldb: i32, stride_b: i64,
+        beta: *const c_void,
+        c: *mut c_void, c_type: i32, ldc: i32, stride_c: i64,
+        batch_count: i32,
+        compute_type: i32,
+        algo: i32,
+    ) -> i32;
 }
 
 pub struct CublasContext {
@@ -149,3 +162,68 @@ pub fn matmul(a: &Tensor, b: &Tensor, backend: GemmBackend) -> Tensor {
 
     c
 }
+
+/// Batched matrix multiplication via cuBLAS: C[b] = A[b] @ B[b]
+/// a: [..., M, K], b: [..., K, N] → [..., M, N]
+/// Leading dimensions must match and tensors must be contiguous.
+pub fn batched_matmul(a: &Tensor, b: &Tensor) -> Tensor {
+    assert!(a.ndim() >= 2 && b.ndim() >= 2);
+    assert_eq!(a.ndim(), b.ndim());
+    assert!(a.is_contiguous() && b.is_contiguous());
+    assert!(matches!(a.device(), Device::Cuda(_)));
+    assert_eq!(a.dtype(), b.dtype());
+
+    let ndim = a.ndim();
+    let m = a.shape()[ndim - 2];
+    let k = a.shape()[ndim - 1];
+    let n = b.shape()[ndim - 1];
+    assert_eq!(b.shape()[ndim - 2], k, "inner dimension mismatch");
+
+    // Compute batch count from leading dimensions
+    let batch: usize = a.shape()[..ndim - 2].iter().product();
+    assert_eq!(
+        b.shape()[..ndim - 2].iter().product::<usize>(),
+        batch,
+        "batch dimensions mismatch"
+    );
+
+    let mut out_shape: Vec<usize> = a.shape()[..ndim - 2].to_vec();
+    out_shape.push(m);
+    out_shape.push(n);
+    let c = Tensor::zeros(&out_shape, a.dtype(), a.device());
+
+    let dtype = a.dtype();
+    let (a_type, b_type, c_type) = match dtype {
+        DType::F32 => (CUDA_R_32F, CUDA_R_32F, CUDA_R_32F),
+        DType::BF16 => (CUDA_R_16BF, CUDA_R_16BF, CUDA_R_16BF),
+        _ => panic!("unsupported dtype for batched matmul"),
+    };
+
+    let alpha = 1.0f32;
+    let beta = 0.0f32;
+    // cuBLAS strides are in elements (not bytes)
+    let stride_a = (m * k) as i64;
+    let stride_b = (k * n) as i64;
+    let stride_c = (m * n) as i64;
+
+    let ctx = CublasContext::new().unwrap();
+    unsafe {
+        cublasSetStream_v2(ctx.handle, std::ptr::null_mut());
+        // Row-major trick: C = A @ B ⟺ C^T = B^T @ A^T (col-major)
+        error::check(cublasGemmStridedBatchedEx(
+            ctx.handle,
+            CUBLAS_OP_N, CUBLAS_OP_N,
+            n as i32, m as i32, k as i32,
+            &alpha as *const f32 as *const c_void,
+            b.data_ptr() as _, b_type, n as i32, stride_b,
+            a.data_ptr() as _, a_type, k as i32, stride_a,
+            &beta as *const f32 as *const c_void,
+            c.data_ptr() as *mut c_void, c_type, n as i32, stride_c,
+            batch as i32,
+            CUBLAS_COMPUTE_32F,
+            -1,
+        )).expect("cuBLAS batched GEMM failed");
+    }
+    xserv_cuda::device::synchronize().unwrap();
+    c
+}

crates/xserv-kernels/src/lib.rs

@@ -1,4 +1,5 @@
 pub mod activation;
+pub mod attention;
 pub mod embedding;
 pub mod gemm;
 pub mod layernorm;
@@ -6,9 +7,10 @@ pub mod rmsnorm;
 pub mod rope;
 pub mod softmax;
 
-pub use activation::{gelu, silu};
+pub use activation::{gelu, scale, silu};
+pub use attention::attention;
 pub use embedding::embedding;
-pub use gemm::{matmul, GemmBackend};
+pub use gemm::{batched_matmul, matmul, GemmBackend};
 pub use layernorm::layernorm;
 pub use rmsnorm::rmsnorm;
 pub use rope::{rope_inplace, RopeCache};