From bd8bd52308ea40c0d534d17969110b03e935a382 Mon Sep 17 00:00:00 2001
From: Itay Etelis <92247226+Etelis@users.noreply.github.com>
Date: Sat, 11 Apr 2026 20:02:34 +0300
Subject: [PATCH] [Bugfix] Runtime driver check for cuMemcpyBatchAsync in
 swap_blocks_batch (#38919)

Signed-off-by: Itay Etelis <itay.etelis@ibm.com>
Co-authored-by: Itay Etelis <itay.etelis@ibm.com>
Co-authored-by: Michael Goin <mgoin64@gmail.com>
---
 csrc/cache_kernels.cu | 72 +++++++++++++++++++++++++------------------
 1 file changed, 42 insertions(+), 30 deletions(-)

diff --git a/csrc/cache_kernels.cu b/csrc/cache_kernels.cu
index a7e5fdabf6..d1cfbaeb05 100644
--- a/csrc/cache_kernels.cu
+++ b/csrc/cache_kernels.cu
@@ -104,37 +104,49 @@ void swap_blocks_batch(const torch::Tensor& src_ptrs,
   static_assert(sizeof(CUdeviceptr) == sizeof(int64_t));
   static_assert(sizeof(size_t) == sizeof(int64_t));
 #if !defined(USE_ROCM) && defined(CUDA_VERSION) && CUDA_VERSION >= 12080
-  CUmemcpyAttributes attr = {};
-  attr.srcAccessOrder = CU_MEMCPY_SRC_ACCESS_ORDER_STREAM;
-  size_t attrs_idx = 0;
-  #if defined(CUDA_VERSION) && CUDA_VERSION >= 13000
-  CUresult result = cuMemcpyBatchAsync(
-      reinterpret_cast<CUdeviceptr*>(dst_data),
-      reinterpret_cast<CUdeviceptr*>(src_data),
-      reinterpret_cast<size_t*>(size_data), static_cast<size_t>(n), &attr,
-      &attrs_idx, 1, static_cast<CUstream>(stream));
-  TORCH_CHECK(result == CUDA_SUCCESS, "cuMemcpyBatchAsync failed with error ",
-              result);
-  #else
-  size_t fail_idx = 0;
-  CUresult result = cuMemcpyBatchAsync(
-      reinterpret_cast<CUdeviceptr*>(dst_data),
-      reinterpret_cast<CUdeviceptr*>(src_data),
-      reinterpret_cast<size_t*>(size_data), static_cast<size_t>(n), &attr,
-      &attrs_idx, 1, &fail_idx, static_cast<CUstream>(stream));
-  TORCH_CHECK(result == CUDA_SUCCESS, "cuMemcpyBatchAsync failed at index ",
-              fail_idx, " with error ", result);
-  #endif
-#else
-  // Fallback for CUDA < 12.8 and ROCm: individual async copies.
-  // cudaMemcpyDefault lets the driver infer direction from pointer types.
-  for (int64_t i = 0; i < n; i++) {
-    cudaMemcpyAsync(reinterpret_cast<void*>(dst_data[i]),
-                    reinterpret_cast<void*>(src_data[i]),
-                    static_cast<size_t>(size_data[i]), cudaMemcpyDefault,
-                    stream);
-  }
+  // Resolve cuMemcpyBatchAsync at runtime via cuGetProcAddress so that
+  // binaries compiled with CUDA 12.8+ still work on older drivers, and
+  // we avoid the CUDA 13.0 header remapping (#define to _v2 signature).
+  // The function pointer is cached after the first call.
+  using BatchFn =
+      CUresult (*)(CUdeviceptr*, CUdeviceptr*, size_t*, size_t,
+                   CUmemcpyAttributes*, size_t*, size_t, size_t*, CUstream);
+  static BatchFn batch_fn = []() -> BatchFn {
+    CUdriverProcAddressQueryResult sym_status;
+    void* fn_ptr = nullptr;
+    CUresult res = cuGetProcAddress("cuMemcpyBatchAsync", &fn_ptr, 12080,
+                                    CU_GET_PROC_ADDRESS_DEFAULT, &sym_status);
+    if (res != CUDA_SUCCESS || fn_ptr == nullptr) {
+      return nullptr;
+    }
+    return reinterpret_cast<BatchFn>(fn_ptr);
+  }();
+
+  if (batch_fn != nullptr) {
+    CUmemcpyAttributes attr = {};
+    attr.srcAccessOrder = CU_MEMCPY_SRC_ACCESS_ORDER_STREAM;
+    size_t attrs_idx = 0;
+    size_t fail_idx = 0;
+    CUresult result = batch_fn(reinterpret_cast<CUdeviceptr*>(dst_data),
+                               reinterpret_cast<CUdeviceptr*>(src_data),
+                               reinterpret_cast<size_t*>(size_data),
+                               static_cast<size_t>(n), &attr, &attrs_idx, 1,
+                               &fail_idx, static_cast<CUstream>(stream));
+    TORCH_CHECK(result == CUDA_SUCCESS, "cuMemcpyBatchAsync failed at index ",
+                fail_idx, " with error ", result);
+  } else
 #endif
+  {
+    // Fallback for CUDA < 12.8, older drivers, and ROCm:
+    // individual async copies.
+    // cudaMemcpyDefault lets the driver infer direction from pointer types.
+    for (int64_t i = 0; i < n; i++) {
+      cudaMemcpyAsync(reinterpret_cast<void*>(dst_data[i]),
+                      reinterpret_cast<void*>(src_data[i]),
+                      static_cast<size_t>(size_data[i]), cudaMemcpyDefault,
+                      stream);
+    }
+  }
 }
 
 namespace vllm {