[CPU][Perf] Accelerate Attention head for s390x using vector intrinsics (#34434)

Signed-off-by: Rehan Khan <Rehan.Khan7@ibm.com>
Co-authored-by: Li, Jiang <jiang1.li@intel.com>

csrc/cpu/cpu_attn.cpp

@@ -16,6 +16,8 @@ torch::Tensor get_scheduler_metadata(
     isa = cpu_attention::ISA::VEC16;
   } else if (isa_hint == "neon") {
     isa = cpu_attention::ISA::NEON;
+  } else if (isa_hint == "vxe") {
+    isa = cpu_attention::ISA::VXE;
   } else {
     TORCH_CHECK(false, "Unsupported CPU attention ISA hint: " + isa_hint);
   }
@@ -100,6 +102,8 @@ void cpu_attn_reshape_and_cache(
       return cpu_attention::ISA::VEC16;
     } else if (isa == "neon") {
       return cpu_attention::ISA::NEON;
+    } else if (isa == "vxe") {
+      return cpu_attention::ISA::VXE;
     } else {
       TORCH_CHECK(false, "Invalid ISA type: " + isa);
     }

csrc/cpu/cpu_attn_impl.hpp

@@ -12,7 +12,7 @@
 #include "cpu/utils.hpp"
 
 namespace cpu_attention {
-enum class ISA { AMX, VEC, VEC16, NEON };
+enum class ISA { AMX, VEC, VEC16, NEON, VXE };
 
 template <ISA isa, typename scalar_t, int64_t head_dim>
 class AttentionImpl {};

csrc/cpu/cpu_attn_vxe.hpp

+#ifndef CPU_ATTN_VXE_HPP
+#define CPU_ATTN_VXE_HPP
+
+#include "cpu_attn_impl.hpp"
+#include <vecintrin.h>
+#include <type_traits>
+
+namespace cpu_attention {
+
+namespace {
+
+// s390x Vector = 16 bytes (128 bits)
+#define BLOCK_SIZE_ALIGNMENT 32
+#define HEAD_SIZE_ALIGNMENT 32
+#define MAX_Q_HEAD_NUM_PER_ITER 16
+
+template <typename kv_cache_t>
+FORCE_INLINE void load_row8_B_as_f32(const kv_cache_t* p, __vector float& b0,
+                                     __vector float& b1);
+
+// [1] Float Specialization
+template <>
+FORCE_INLINE void load_row8_B_as_f32<float>(const float* p, __vector float& b0,
+                                            __vector float& b1) {
+  // Explicitly cast to long long for offset, and float* for pointer
+  b0 = vec_xl((long long)0, const_cast<float*>(p));
+  b1 = vec_xl((long long)0, const_cast<float*>(p + 4));
+}
+
+// [2] BFloat16 Specialization (Big Endian Fix)
+template <>
+FORCE_INLINE void load_row8_B_as_f32<c10::BFloat16>(const c10::BFloat16* p,
+                                                    __vector float& b0,
+                                                    __vector float& b1) {
+  // 1. Load 8 BF16s (16 bytes) into one vector
+  // Explicit cast to unsigned short* for vec_xl to return vector unsigned short
+  __vector unsigned short raw = vec_xl((long long)0, (unsigned short*)p);
+
+  // 2. Prepare Zero vector
+  __vector unsigned short zeros = vec_splat_u16(0);
+
+  // 3. Merge High/Low to expand BF16 -> Float32
+  // On Big Endian, a float is [BF16_bits | 16_zero_bits]
+  b0 = (__vector float)vec_mergeh(raw, zeros);
+  b1 = (__vector float)vec_mergel(raw, zeros);
+}
+
+template <>
+FORCE_INLINE void load_row8_B_as_f32<c10::Half>(const c10::Half* p,
+                                                __vector float& b0,
+                                                __vector float& b1) {
+  alignas(16) float tmp[8];
+
+  // Manual unroll / conversion
+  tmp[0] = static_cast<float>(p[0]);
+  tmp[1] = static_cast<float>(p[1]);
+  tmp[2] = static_cast<float>(p[2]);
+  tmp[3] = static_cast<float>(p[3]);
+  tmp[4] = static_cast<float>(p[4]);
+  tmp[5] = static_cast<float>(p[5]);
+  tmp[6] = static_cast<float>(p[6]);
+  tmp[7] = static_cast<float>(p[7]);
+
+  // Explicit arguments for intrinsic: (long long offset, float* ptr)
+  b0 = vec_xl((long long)0, (float*)tmp);
+  b1 = vec_xl((long long)0, (float*)(tmp + 4));
+}
+
+template <int32_t M, typename kv_cache_t>
+FORCE_INLINE void gemm_micro_s390x_Mx8_Ku4(
+    const float* __restrict A,       // [M x K]
+    const kv_cache_t* __restrict B,  // [K x 8]
+    float* __restrict C,             // [M x 8]
+    int64_t lda, int64_t ldb, int64_t ldc, int32_t K, bool accumulate) {
+  static_assert(1 <= M && M <= 8, "M must be in [1,8]");
+
+// Helper macros to unroll codegen for M rows
+#define ROWS_APPLY(OP) OP(0) OP(1) OP(2) OP(3) OP(4) OP(5) OP(6) OP(7)
+#define IF_M(i) if constexpr (M > (i))
+
+  // 1. Define A pointers
+#define DECL_A(i) const float* a##i = A + (i) * lda;
+  ROWS_APPLY(DECL_A)
+#undef DECL_A
+
+  // 2. Define Accumulators (2 vectors covers 8 columns)
+#define DECL_ACC(i) __vector float acc##i##_0, acc##i##_1;
+  ROWS_APPLY(DECL_ACC)
+#undef DECL_ACC
+
+  // 3. Initialize Accumulators (Load C or Zero)
+#define INIT_ACC(i)                                                    \
+  IF_M(i) {                                                            \
+    if (accumulate) {                                                  \
+      acc##i##_0 =                                                     \
+          vec_xl((long long)0, const_cast<float*>(C + (i) * ldc + 0)); \
+      acc##i##_1 =                                                     \
+          vec_xl((long long)0, const_cast<float*>(C + (i) * ldc + 4)); \
+    } else {                                                           \
+      acc##i##_0 = vec_splats(0.0f);                                   \
+      acc##i##_1 = vec_splats(0.0f);                                   \
+    }                                                                  \
+  }
+  ROWS_APPLY(INIT_ACC)
+#undef INIT_ACC
+
+  int32_t k = 0;
+
+  for (; k + 3 < K; k += 4) {
+    // Load 4 values of A for each Row M: A[k...k+3]
+#define LOAD_A4(i)        \
+  __vector float a##i##v; \
+  IF_M(i) a##i##v = vec_xl((long long)0, const_cast<float*>(a##i + k));
+    ROWS_APPLY(LOAD_A4)
+#undef LOAD_A4
+
+    // Helper: FMA for specific lane L of A
+    // s390x: vec_madd(b, vec_splat(a, lane), acc)
+#define FMAS_LANE(i, aiv, L)                        \
+  IF_M(i) {                                         \
+    __vector float a_broad = vec_splat(aiv, L);     \
+    acc##i##_0 = vec_madd(b0, a_broad, acc##i##_0); \
+    acc##i##_1 = vec_madd(b1, a_broad, acc##i##_1); \
+  }
+
+    // Unroll K=0..3
+    {
+      __vector float b0, b1;
+      load_row8_B_as_f32<kv_cache_t>(B + (int64_t)(k + 0) * ldb, b0, b1);
+#define STEP_K0(i) FMAS_LANE(i, a##i##v, 0)
+      ROWS_APPLY(STEP_K0)
+#undef STEP_K0
+    }
+    {
+      __vector float b0, b1;
+      load_row8_B_as_f32<kv_cache_t>(B + (int64_t)(k + 1) * ldb, b0, b1);
+#define STEP_K1(i) FMAS_LANE(i, a##i##v, 1)
+      ROWS_APPLY(STEP_K1)
+#undef STEP_K1
+    }
+    {
+      __vector float b0, b1;
+      load_row8_B_as_f32<kv_cache_t>(B + (int64_t)(k + 2) * ldb, b0, b1);
+#define STEP_K2(i) FMAS_LANE(i, a##i##v, 2)
+      ROWS_APPLY(STEP_K2)
+#undef STEP_K2
+    }
+
+    {
+      __vector float b0, b1;
+      load_row8_B_as_f32<kv_cache_t>(B + (int64_t)(k + 3) * ldb, b0, b1);
+#define STEP_K3(i) FMAS_LANE(i, a##i##v, 3)
+      ROWS_APPLY(STEP_K3)
+#undef STEP_K3
+    }
+#undef FMAS_LANE
+  }
+
+  for (; k < K; ++k) {
+    __vector float b0, b1;
+    load_row8_B_as_f32<kv_cache_t>(B + (int64_t)k * ldb, b0, b1);
+#define TAIL_ROW(i)                              \
+  IF_M(i) {                                      \
+    __vector float ai = vec_splats(*(a##i + k)); \
+    acc##i##_0 = vec_madd(b0, ai, acc##i##_0);   \
+    acc##i##_1 = vec_madd(b1, ai, acc##i##_1);   \
+  }
+    ROWS_APPLY(TAIL_ROW)
+#undef TAIL_ROW
+  }
+
+#define STORE_ROW(i)                           \
+  IF_M(i) {                                    \
+    vec_xst(acc##i##_0, 0, C + (i) * ldc + 0); \
+    vec_xst(acc##i##_1, 0, C + (i) * ldc + 4); \
+  }
+  ROWS_APPLY(STORE_ROW)
+#undef STORE_ROW
+
+#undef ROWS_APPLY
+#undef IF_M
+}
+
+template <int32_t N, typename kv_cache_t>
+FORCE_INLINE void gemm_macro_s390x_Mx8_Ku4(const float* __restrict A,
+                                           const kv_cache_t* __restrict B,
+                                           float* __restrict C, int32_t M,
+                                           int32_t K, int64_t lda, int64_t ldb,
+                                           int64_t ldc, bool accumulate) {
+  static_assert(N % 8 == 0, "N must be a multiple of 8");
+  for (int32_t m = 0; m < M;) {
+    int32_t mb = (M - m >= 8) ? 8 : (M - m >= 4) ? 4 : (M - m >= 2) ? 2 : 1;
+    const float* Ab = A + m * lda;
+    float* Cb = C + m * ldc;
+
+    for (int32_t n = 0; n < N; n += 8) {
+      const kv_cache_t* Bn = B + n;
+      float* Cn = Cb + n;
+      switch (mb) {
+        case 8:
+          gemm_micro_s390x_Mx8_Ku4<8, kv_cache_t>(Ab, Bn, Cn, lda, ldb, ldc, K,
+                                                  accumulate);
+          break;
+        case 4:
+          gemm_micro_s390x_Mx8_Ku4<4, kv_cache_t>(Ab, Bn, Cn, lda, ldb, ldc, K,
+                                                  accumulate);
+          break;
+        case 2:
+          gemm_micro_s390x_Mx8_Ku4<2, kv_cache_t>(Ab, Bn, Cn, lda, ldb, ldc, K,
+                                                  accumulate);
+          break;
+        default:
+          gemm_micro_s390x_Mx8_Ku4<1, kv_cache_t>(Ab, Bn, Cn, lda, ldb, ldc, K,
+                                                  accumulate);
+          break;
+      }
+    }
+    m += mb;
+  }
+}
+
+template <typename kv_cache_t>
+class TileGemmS390X {
+ public:
+  template <AttentionGemmPhase phase, int32_t k_size>
+  FORCE_INLINE static void gemm(const int32_t m_size,
+                                float* __restrict__ a_tile,
+                                kv_cache_t* __restrict__ b_tile,
+                                float* __restrict__ c_tile, const int64_t lda,
+                                const int64_t ldb, const int64_t ldc,
+                                const int32_t block_size,
+                                const int32_t dynamic_k_size,
+                                const bool accum_c) {
+    if constexpr (phase == AttentionGemmPhase::QK) {
+      gemm_macro_s390x_Mx8_Ku4<BLOCK_SIZE_ALIGNMENT, kv_cache_t>(
+          a_tile, b_tile, c_tile, m_size, k_size, lda, ldb, ldc, accum_c);
+    } else {
+      gemm_macro_s390x_Mx8_Ku4<HEAD_SIZE_ALIGNMENT, kv_cache_t>(
+          a_tile, b_tile, c_tile, m_size, dynamic_k_size, lda, ldb, ldc,
+          accum_c);
+    }
+  }
+};
+
+}  // namespace
+
+template <typename scalar_t, int64_t head_dim>
+class AttentionImpl<ISA::VXE, scalar_t, head_dim> {
+ public:
+  using query_t = scalar_t;
+  using q_buffer_t = float;
+  using kv_cache_t = scalar_t;
+  using logits_buffer_t = float;
+  using partial_output_buffer_t = float;
+  using prob_buffer_t = float;
+
+  constexpr static int64_t BlockSizeAlignment = BLOCK_SIZE_ALIGNMENT;
+  constexpr static int64_t HeadDimAlignment = HEAD_SIZE_ALIGNMENT;
+  constexpr static int64_t MaxQHeadNumPerIteration = MAX_Q_HEAD_NUM_PER_ITER;
+  constexpr static int64_t HeadDim = head_dim;
+  constexpr static ISA ISAType = ISA::VXE;
+  constexpr static bool scale_on_logits =
+      false;  // Scale is applied to Q during copy
+
+ public:
+  AttentionImpl() {}
+
+  template <template <typename tile_gemm_t> typename attention>
+  FORCE_INLINE void execute_attention(DEFINE_CPU_ATTENTION_PARAMS) {
+    attention<TileGemmS390X<kv_cache_t>> attention_iteration;
+    attention_iteration(CPU_ATTENTION_PARAMS);
+  }
+
+  // Strides for Memory Layout
+  constexpr static int64_t k_cache_token_group_stride(
+      const int32_t block_size) {
+    return BlockSizeAlignment;  // [head_dim, block_size] layout
+  }
+
+  constexpr static int64_t v_cache_token_group_stride(
+      const int32_t block_size) {
+    return head_dim * BlockSizeAlignment;
+  }
+
+  constexpr static int64_t v_cache_head_group_stride(const int32_t block_size) {
+    return HeadDimAlignment;
+  }
+
+  static void copy_q_heads_tile(scalar_t* __restrict__ src,
+                                float* __restrict__ q_buffer,
+                                const int32_t q_num,
+                                const int32_t q_heads_per_kv,
+                                const int64_t q_num_stride,
+                                const int64_t q_head_stride, float scale) {
+    __vector float scale_vec = vec_splats(scale);
+    constexpr bool is_bf16 = std::is_same<scalar_t, c10::BFloat16>::value;
+
+    // Process 8 elements at a time (32 bytes of float output)
+    for (int32_t i = 0; i < q_num; ++i) {
+      for (int32_t h = 0; h < q_heads_per_kv; ++h) {
+        scalar_t* curr_src = src + i * q_num_stride + h * q_head_stride;
+        float* curr_dst =
+            q_buffer + i * q_heads_per_kv * head_dim + h * head_dim;
+
+        int32_t d = 0;
+        for (; d <= head_dim - 8; d += 8) {
+          if constexpr (is_bf16) {
+            __vector float v0, v1;
+            // Reuse our Big-Endian-Safe loader
+            load_row8_B_as_f32<scalar_t>(curr_src + d, v0, v1);
+
+            v0 = vec_mul(v0, scale_vec);
+            v1 = vec_mul(v1, scale_vec);
+
+            vec_xst(v0, 0, curr_dst + d);
+            vec_xst(v1, 0, curr_dst + d + 4);
+          } else {
+            __vector float v0 = vec_xl((long long)0, (float*)curr_src + d);
+            __vector float v1 = vec_xl((long long)0, (float*)curr_src + d + 4);
+
+            v0 = vec_mul(v0, scale_vec);
+            v1 = vec_mul(v1, scale_vec);
+
+            vec_xst(v0, 0, curr_dst + d);
+            vec_xst(v1, 0, curr_dst + d + 4);
+          }
+        }
+
+        for (; d < head_dim; ++d) {
+          float val = static_cast<float>(curr_src[d]);
+          curr_dst[d] = val * scale;
+        }
+      }
+    }
+  }
+
+  static void reshape_and_cache(
+      const scalar_t* __restrict__ key, const scalar_t* __restrict__ value,
+      scalar_t* __restrict__ key_cache, scalar_t* __restrict__ value_cache,
+      const int64_t* __restrict__ slot_mapping, const int64_t token_num,
+      const int64_t key_token_num_stride, const int64_t value_token_num_stride,
+      const int64_t head_num, const int64_t key_head_num_stride,
+      const int64_t value_head_num_stride, const int64_t num_blocks,
+      const int64_t num_blocks_stride, const int64_t cache_head_num_stride,
+      const int64_t block_size, const int64_t block_size_stride) {
+#pragma omp parallel for collapse(2)
+    for (int64_t token_idx = 0; token_idx < token_num; ++token_idx) {
+      for (int64_t head_idx = 0; head_idx < head_num; ++head_idx) {
+        const int64_t pos = slot_mapping[token_idx];
+        if (pos < 0) continue;
+
+        const int64_t block_idx = pos / block_size;
+        const int64_t block_offset = pos % block_size;
+
+        {
+          const scalar_t* key_src = key + token_idx * key_token_num_stride +
+                                    head_idx * key_head_num_stride;
+          scalar_t* key_dst = key_cache + block_idx * num_blocks_stride +
+                              head_idx * cache_head_num_stride + block_offset;
+
+          for (int64_t i = 0, j = 0; i < head_dim; ++i, j += block_size) {
+            key_dst[j] = key_src[i];
+          }
+        }
+
+        {
+          const scalar_t* val_src = value + token_idx * value_token_num_stride +
+                                    head_idx * value_head_num_stride;
+          scalar_t* val_dst = value_cache + block_idx * num_blocks_stride +
+                              head_idx * cache_head_num_stride +
+                              block_offset * head_dim;
+
+          std::memcpy(val_dst, val_src, sizeof(scalar_t) * head_dim);
+        }
+      }
+    }
+  }
+};
+
+}  // namespace cpu_attention
+
+#undef BLOCK_SIZE_ALIGNMENT
+#undef HEAD_SIZE_ALIGNMENT
+#undef MAX_Q_HEAD_NUM_PER_ITER
+
+#endif
\ No newline at end of file

csrc/cpu/generate_cpu_attn_dispatch.py

@@ -19,10 +19,11 @@ ISA_TYPES = {
     "VEC": 1,
     "VEC16": 2,
     "NEON": 3,
+    "VXE": 4,
 }
 
 # ISAs supported for head_dims divisible by 32
-ISA_FOR_32 = ["AMX", "NEON", "VEC", "VEC16"]
+ISA_FOR_32 = ["AMX", "NEON", "VEC", "VEC16", "VXE"]
 
 # ISAs supported for head_dims divisible by 16 only
 ISA_FOR_16 = ["VEC16"]
@@ -118,6 +119,10 @@ def generate_header_file() -> str:
   #include "cpu_attn_neon.hpp"
 #endif
 
+#ifdef __s390x__
+  #include "cpu_attn_vxe.hpp"
+#endif
+
 """
 
     header += generate_helper_function()
@@ -163,6 +168,25 @@ def generate_header_file() -> str:
     } \\
   }()
 
+"""
+
+    # s390x with VXE
+    header += """#elif defined(__s390x__)
+#define CPU_ATTN_DISPATCH(HEAD_DIM, ISA_TYPE, ...) \\
+  [&] { \\
+    int64_t encoded_params = encode_cpu_attn_params(HEAD_DIM, ISA_TYPE); \\
+    switch (encoded_params) { \\
+"""
+    header += generate_cases_for_isa_group(["VXE", "VEC", "VEC16"])
+    header += """
+      default: { \\
+        TORCH_CHECK(false, "Unsupported CPU attention configuration: head_dim=" + \\
+                    std::to_string(HEAD_DIM) + " isa=" + \\
+                    std::to_string(static_cast<int>(ISA_TYPE))); \\
+      } \\
+    } \\
+  }()
+
 """
 
     # Fallback: VEC and VEC16 only
@@ -182,7 +206,7 @@ def generate_header_file() -> str:
     } \\
   }()
 
-#endif  /* CPU_CAPABILITY_AMXBF16 / __aarch64__ */
+#endif  /* CPU_CAPABILITY_AMXBF16 / __aarch64__ / __s390x__ */
 
 #endif  // CPU_ATTN_DISPATCH_GENERATED_H
 """

vllm/engine/arg_utils.py

@@ -2017,21 +2017,20 @@ class EngineArgs:
             )
 
         # Disable chunked prefill and prefix caching for:
-        # POWER (ppc64le)/s390x/RISCV CPUs in V1
+        # POWER (ppc64le)/RISCV CPUs in V1
         if current_platform.is_cpu() and current_platform.get_cpu_architecture() in (
             CpuArchEnum.POWERPC,
-            CpuArchEnum.S390X,
             CpuArchEnum.RISCV,
         ):
             logger.info(
                 "Chunked prefill is not supported for POWER, "
-                "S390X and RISC-V CPUs; "
+                "and RISC-V CPUs; "
                 "disabling it for V1 backend."
             )
             self.enable_chunked_prefill = False
             logger.info(
                 "Prefix caching is not supported for POWER, "
-                "S390X and RISC-V CPUs; "
+                "and RISC-V CPUs; "
                 "disabling it for V1 backend."
             )
             self.enable_prefix_caching = False

vllm/v1/attention/backends/cpu_attn.py

@@ -25,7 +25,7 @@ from vllm.v1.kv_cache_interface import AttentionSpec, CrossAttentionSpec
 
 logger = init_logger(__name__)
 
-_CPU_ARCH_PREFER_MIXED_BATCH = (CpuArchEnum.X86, CpuArchEnum.ARM)
+_CPU_ARCH_PREFER_MIXED_BATCH = (CpuArchEnum.X86, CpuArchEnum.ARM, CpuArchEnum.S390X)
 
 
 class CPUAttentionBackend(AttentionBackend):
@@ -488,12 +488,15 @@ def _get_attn_isa(
         return "vec16"
     supports_amx = torch._C._cpu._is_amx_tile_supported()
     supports_arm = current_platform.get_cpu_architecture() == CpuArchEnum.ARM
+    supports_vxe = current_platform.get_cpu_architecture() == CpuArchEnum.S390X
     if supports_amx and dtype in (torch.bfloat16,) and block_size % 32 == 0:
         return "amx"
     elif block_size % 32 == 0:
         if supports_arm:
             # support ARM NEON FMLA and BFMMLA (bf16) for block size 32
             return "neon"
+        elif supports_vxe:
+            return "vxe"
         else:
             return "vec"
     else: