[STF] Add exec places from externally-owned CUDA contexts

c/experimental/stf/include/cccl/c/experimental/stf/stf.h

@@ -161,6 +161,15 @@ stf_exec_place_handle stf_exec_place_device(int dev_id);
 //! \brief Create execution place for the current CUDA device.
 stf_exec_place_handle stf_exec_place_current_device(void);
 
+//! \brief Create an execution place from an externally-owned CUDA driver context \p ctx.
+//!
+//! The place is non-owning: the caller must keep \p ctx alive while the place is in
+//! use. This is the natural entry point for contexts created by other libraries, e.g.
+//! green contexts converted with cuCtxFromGreenCtx (such as the ones produced by
+//! cuda.core in Python). \p dev_id is the device ordinal of the context, or -1 to
+//! derive it from the context. Returns NULL on failure.
+stf_exec_place_handle stf_exec_place_cuda_context(CUcontext ctx, int dev_id);
+
 //! \brief Create a green-context helper for \p dev_id with \p sm_count SMs per green context.
 //! Requires CUDA 12.4+. Returns NULL on failure.
 stf_green_context_helper_handle stf_green_context_helper_create(int sm_count, int dev_id);

c/experimental/stf/src/stf.cu

@@ -219,6 +219,17 @@ stf_exec_place_handle stf_exec_place_current_device(void)
   }));
 }
 
+stf_exec_place_handle stf_exec_place_cuda_context(CUcontext ctx, int dev_id)
+{
+  if (ctx == nullptr)
+  {
+    return nullptr;
+  }
+  return to_opaque(stf_try_allocate([ctx, dev_id] {
+    return new exec_place(exec_place::cuda_context(ctx, dev_id));
+  }));
+}
+
 stf_green_context_helper_handle stf_green_context_helper_create(int sm_count, int dev_id)
 {
 #if _CCCL_CTK_AT_LEAST(12, 4)

c/experimental/stf/test/test_places.cpp

@@ -41,6 +41,63 @@ static void blocked_mapper_1d(stf_pos4* result, stf_pos4 data_coords, stf_dim4 d
   result->t = 0;
 }
 
+C2H_TEST("exec place from an externally-owned CUDA context", "[task][places][cuda_context]")
+{
+  const size_t N = 1024;
+
+  // Wrap the primary context of device 0 as an exec place
+  CUdevice dev = 0;
+  REQUIRE(cuDeviceGet(&dev, 0) == CUDA_SUCCESS);
+  CUcontext primary_ctx = nullptr;
+  REQUIRE(cuDevicePrimaryCtxRetain(&primary_ctx, dev) == CUDA_SUCCESS);
+
+  // Null context is rejected
+  REQUIRE(stf_exec_place_cuda_context(nullptr, 0) == nullptr);
+
+  // dev_id < 0 is derived from the context
+  stf_exec_place_handle place_derived = stf_exec_place_cuda_context(primary_ctx, -1);
+  REQUIRE(place_derived != nullptr);
+  REQUIRE(stf_exec_place_is_device(place_derived) != 0);
+  stf_exec_place_destroy(place_derived);
+
+  stf_exec_place_handle place = stf_exec_place_cuda_context(primary_ctx, 0);
+  REQUIRE(place != nullptr);
+  REQUIRE(stf_exec_place_is_device(place) != 0);
+  REQUIRE(stf_exec_place_is_host(place) == 0);
+
+  // Run a task on the place and fill the buffer through its stream
+  stf_ctx_handle ctx = stf_ctx_create();
+  REQUIRE(ctx != nullptr);
+
+  std::vector<float> X(N, 1.0f);
+  stf_logical_data_handle lX = stf_logical_data(ctx, X.data(), N * sizeof(float));
+  REQUIRE(lX != nullptr);
+
+  stf_task_handle t = stf_task_create(ctx);
+  REQUIRE(t != nullptr);
+  stf_task_set_exec_place(t, place);
+  stf_task_add_dep(t, lX, STF_RW);
+  stf_task_start(t);
+  CUstream stream = stf_task_get_custream(t);
+  REQUIRE(stream != nullptr);
+  float* dX = static_cast<float*>(stf_task_get(t, 0));
+  REQUIRE(dX != nullptr);
+  REQUIRE(cudaMemsetAsync(dX, 0, N * sizeof(float), stream) == cudaSuccess);
+  stf_task_end(t);
+  stf_task_destroy(t);
+
+  stf_logical_data_destroy(lX);
+  stf_ctx_finalize(ctx);
+
+  for (size_t i = 0; i < N; i++)
+  {
+    REQUIRE(X[i] == 0.0f);
+  }
+
+  stf_exec_place_destroy(place);
+  REQUIRE(cuDevicePrimaryCtxRelease(dev) == CUDA_SUCCESS);
+}
+
 C2H_TEST("empty stf tasks", "[task]")
 {
   size_t N = 1000000;

cudax/include/cuda/experimental/__places/exec/cuda_context.cuh

@@ -0,0 +1,253 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of CUDASTF in CUDA C++ Core Libraries,
+// under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+// SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES.
+//
+//===----------------------------------------------------------------------===//
+
+/**
+ * @file
+ * @brief Execution place wrapping an externally-owned CUDA driver context
+ *
+ * This makes it possible to use a CUcontext created outside CUDASTF (e.g. a
+ * green context created through cuda.core in Python, or any other library)
+ * as an execution place. The place is non-owning: the caller must keep the
+ * context alive while the place (and the streams lazily created in its pool)
+ * is in use.
+ */
+
+#pragma once
+
+#include <cuda/__cccl_config>
+
+#if defined(_CCCL_IMPLICIT_SYSTEM_HEADER_GCC)
+#  pragma GCC system_header
+#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_CLANG)
+#  pragma clang system_header
+#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_MSVC)
+#  pragma system_header
+#endif // no system header
+
+#include <cuda/experimental/__places/places.cuh>
+#include <cuda/experimental/__stf/utility/hash.cuh>
+
+namespace cuda::experimental::places
+{
+/**
+ * @brief Implementation for execution places backed by an externally-owned CUcontext
+ *
+ * The context is used as-is: `activate()` saves the current context and makes
+ * this one current, `deactivate()` restores the saved context. Streams are
+ * created lazily in the place's own pool while the context is current, so they
+ * inherit whatever resources the context carries (e.g. the SM partition of a
+ * green context converted with `cuCtxFromGreenCtx`).
+ *
+ * Identity (`hash`/`cmp`) is keyed on the CUcontext handle, which uniquely
+ * identifies the underlying resources: `cuCtxFromGreenCtx` returns the same
+ * CUcontext for a given CUgreenCtx on every call.
+ */
+class exec_place_cuda_ctx_impl : public exec_place::impl
+{
+public:
+  /**
+   * @brief Construct an execution place from an externally-owned CUDA context
+   *
+   * @param ctx The CUDA driver context. Non-owning: the caller keeps it alive.
+   * @param devid The device ordinal the context belongs to, or -1 to derive it
+   *        from the context (via cuCtxGetDevice).
+   * @param pool_size Number of streams in the place's stream pool.
+   */
+  exec_place_cuda_ctx_impl(CUcontext ctx, int devid = -1, size_t pool_size = exec_place::impl::pool_size)
+      : exec_place_cuda_ctx_impl(
+          ctx, resolve_devid(ctx, devid), stream_pool(pool_size), data_place::device(resolve_devid(ctx, devid)))
+  {}
+
+  /**
+   * @brief Full-control constructor, also used by the green-context place
+   *
+   * @param ctx The CUDA driver context (already resolved, e.g. from cuCtxFromGreenCtx)
+   * @param devid The device ordinal (must be valid)
+   * @param pool A stream pool to use for this place (shared handle)
+   * @param affine The affine data place for this place
+   */
+  exec_place_cuda_ctx_impl(CUcontext ctx, int devid, stream_pool pool, data_place affine)
+      : exec_place::impl(mv(affine))
+      , devid_(devid)
+      , driver_context_(ctx)
+      , pool_(mv(pool))
+  {
+    _CCCL_ASSERT(ctx != nullptr, "cuda_ctx exec_place requires a valid CUcontext");
+    _CCCL_ASSERT(devid_ >= 0, "cuda_ctx exec_place requires a valid device ordinal");
+  }
+
+  ::std::shared_ptr<exec_place::impl> get_place(size_t idx) override
+  {
+    _CCCL_ASSERT(idx == 0, "Index out of bounds for cuda_ctx exec_place");
+    return shared_from_this();
+  }
+
+  exec_place activate(size_t idx) const override
+  {
+    _CCCL_ASSERT(idx == 0, "Index out of bounds for cuda_ctx exec_place");
+
+    // Save the current context wrapped as a place so deactivate can restore it
+    CUcontext current_ctx = cuda_try<cuCtxGetCurrent>();
+    exec_place result     = exec_place(::std::make_shared<exec_place_cuda_ctx_impl>(saved_tag{}, current_ctx));
+
+    cuda_try<cuCtxSetCurrent>(driver_context_);
+
+    return result;
+  }
+
+  void deactivate(const exec_place& prev, size_t idx = 0) const override
+  {
+    _CCCL_ASSERT(idx == 0, "Index out of bounds for cuda_ctx exec_place");
+
+    auto prev_impl      = ::std::static_pointer_cast<exec_place_cuda_ctx_impl>(prev.get_impl());
+    CUcontext saved_ctx = prev_impl->driver_context_;
+
+    cuda_try<cuCtxSetCurrent>(saved_ctx);
+  }
+
+  bool is_device() const override
+  {
+    return true;
+  }
+
+  ::std::string to_string() const override
+  {
+    return "cuda_ctx(ctx=" + ::std::to_string(reinterpret_cast<::std::uintptr_t>(driver_context_))
+         + " dev=" + ::std::to_string(devid_) + ")";
+  }
+
+  stream_pool& get_stream_pool(bool, exec_place_resources&, const exec_place&) const override
+  {
+    // This place carries its own pool and bypasses the registry. The user is
+    // responsible for keeping the underlying CUcontext alive while the pool
+    // is in use.
+    return pool_;
+  }
+
+  int cmp(const exec_place::impl& rhs) const override
+  {
+    if (typeid(*this) != typeid(rhs))
+    {
+      return typeid(*this).before(typeid(rhs)) ? -1 : 1;
+    }
+    const auto& other = static_cast<const exec_place_cuda_ctx_impl&>(rhs);
+    return (other.driver_context_ < driver_context_) - (driver_context_ < other.driver_context_);
+  }
+
+  size_t hash() const override
+  {
+    return ::std::hash<CUcontext>()(driver_context_);
+  }
+
+protected:
+  // Tag type for the internal "saved context" wrapper used by activate/deactivate.
+  // The type itself is protected so only this class (and derived classes) can
+  // name it; the constructor below must be public for make_shared.
+  struct saved_tag
+  {};
+
+public:
+  // Wrap an existing context with no pool: only used to carry the saved
+  // context through the activate/deactivate round trip.
+  exec_place_cuda_ctx_impl(saved_tag, CUcontext saved_context)
+      : driver_context_(saved_context)
+  {}
+
+protected:
+  static int resolve_devid(CUcontext ctx, int devid)
+  {
+    if (devid >= 0)
+    {
+      return devid;
+    }
+
+    // Derive the device ordinal from the context
+    cuda_try<cuCtxPushCurrent>(ctx);
+    CUdevice dev                            = cuda_try<cuCtxGetDevice>();
+    [[maybe_unused]] const CUcontext popped = cuda_try<cuCtxPopCurrent>();
+    return static_cast<int>(dev);
+  }
+
+  int devid_                = -1;
+  CUcontext driver_context_ = {};
+  mutable stream_pool pool_;
+};
+
+inline exec_place exec_place::cuda_context(CUcontext ctx, int devid, size_t pool_size)
+{
+  return exec_place(::std::make_shared<exec_place_cuda_ctx_impl>(ctx, devid, pool_size));
+}
+
+#ifdef UNITTESTED_FILE
+namespace
+{
+//! RAII holder for the primary context of device 0, used by the unittests below.
+struct primary_ctx_guard
+{
+  primary_ctx_guard()
+  {
+    cuda_try<cuInit>(0);
+    dev = cuda_try<cuDeviceGet>(0);
+    ctx = cuda_try<cuDevicePrimaryCtxRetain>(dev);
+  }
+
+  ~primary_ctx_guard()
+  {
+    cuda_try(cuDevicePrimaryCtxRelease(dev));
+  }
+
+  CUdevice dev  = -1;
+  CUcontext ctx = nullptr;
+};
+} // namespace
+
+UNITTEST("cuda_context exec_place equality")
+{
+  primary_ctx_guard guard;
+
+  auto p0a = exec_place::cuda_context(guard.ctx, 0);
+  auto p0b = exec_place::cuda_context(guard.ctx, 0);
+
+  // Same context should be equal
+  EXPECT(p0a == p0b);
+  EXPECT(!(p0a != p0b));
+
+  // A cuda_context place is not a regular device place
+  auto dev0 = exec_place::device(0);
+  EXPECT(p0a != dev0);
+  EXPECT(!(p0a == dev0));
+};
+
+UNITTEST("cuda_context exec_place derives the device ordinal")
+{
+  primary_ctx_guard guard;
+
+  // devid intentionally omitted: derived from the context via cuCtxGetDevice
+  auto p = exec_place::cuda_context(guard.ctx);
+  EXPECT(p.is_device());
+  EXPECT(p.affine_data_place() == data_place::device(0));
+  EXPECT(p == exec_place::cuda_context(guard.ctx, 0));
+};
+
+UNITTEST("cuda_context exec_place activate/deactivate round trip")
+{
+  primary_ctx_guard guard;
+
+  auto p = exec_place::cuda_context(guard.ctx, 0);
+
+  CUcontext before = cuda_try<cuCtxGetCurrent>();
+  {
+    exec_place_scope scope(p);
+    EXPECT(cuda_try<cuCtxGetCurrent>() == guard.ctx);
+  }
+  EXPECT(cuda_try<cuCtxGetCurrent>() == before);
+};
+#endif // UNITTESTED_FILE
+} // end namespace cuda::experimental::places