cuda: add per-session mutable state rebinding

Local agent serving needs to host multiple logical conversations on one CUDA-resident model without multiplying the model weights. Loading one AOTI module per conversation is not viable for large local models, while sharing the default mutable state across conversations would let KV/recurrent/conv buffers bleed between users.

This adds the CUDA-private foundation for separating those concerns: weights remain owned by the loaded AOTI container, while mutable buffer FQNs can be registered as per-session state and rebound before execution. The path is fail-closed and dormant until a model opts in by creating a mutable-state context and validating coverage, so existing CUDA models keep their current behavior.

The branch also wires the new source and unit coverage into both Buck and CMake so the primitive can land independently before any model-specific engine consumes it.

Author: mergennachin

backends/cuda/CMakeLists.txt

@@ -184,7 +184,9 @@ install(
 )
 
 # CUDA backend implementation
-set(_aoti_cuda_backend_sources runtime/cuda_backend.cpp)
+set(_aoti_cuda_backend_sources runtime/cuda_backend.cpp
+                               runtime/cuda_mutable_state.cpp
+)
 if(_cuda_is_msvc_toolchain)
   # MSVC links aoti_cuda_backend into portable_lib without relying on C++
   # symbols exported from aoti_cuda_shims.dll.
@@ -236,3 +238,13 @@ install(
   EXPORT ExecuTorchTargets
   DESTINATION lib
 )
+
+if(BUILD_TESTING)
+  include(${EXECUTORCH_ROOT}/tools/cmake/Test.cmake)
+
+  et_cxx_test(
+    test_cuda_mutable_state SOURCES runtime/test/test_cuda_mutable_state.cpp
+    EXTRA_LIBS aoti_cuda_backend
+  )
+  target_compile_definitions(test_cuda_mutable_state PRIVATE CUDA_AVAILABLE=1)
+endif()

backends/cuda/runtime/TARGETS

@@ -1,4 +1,5 @@
 load("@fbsource//xplat/executorch/build:runtime_wrapper.bzl", "runtime")
+load("@fbcode_macros//build_defs:cpp_unittest.bzl", "cpp_unittest")
 load("//tools/build/buck:nvcc_flags.bzl", "get_nvcc_arch_args")
 
 oncall("executorch")
@@ -105,9 +106,11 @@ runtime.cxx_library(
     name = "cuda_backend",
     srcs = [
         "cuda_backend.cpp",
+        "cuda_mutable_state.cpp",
     ],
     headers = [
         "cuda_delegate_handle.h",
+        "cuda_mutable_state.h",
     ],
     # @lint-ignore BUCKLINT: Avoid `link_whole=True` (https://fburl.com/avoid-link-whole)
     link_whole = True,
@@ -135,3 +138,19 @@ runtime.cxx_library(
         ("cuda", None, "cuda-lazy"),
     ],
 )
+
+cpp_unittest(
+    name = "test_cuda_mutable_state",
+    srcs = [
+        "test/test_cuda_mutable_state.cpp",
+    ],
+    deps = [
+        ":cuda_backend",
+        "//executorch/runtime/core:core",
+        "//executorch/runtime/platform:platform",
+    ],
+    external_deps = [
+        ("cuda", None, "cuda-lazy"),
+    ],
+    preprocessor_flags = ["-DCUDA_AVAILABLE=1"],
+)

backends/cuda/runtime/cuda_backend.cpp

@@ -44,6 +44,7 @@
 #include <executorch/backends/aoti/utils.h>
 #include <executorch/backends/cuda/runtime/cuda_allocator.h>
 #include <executorch/backends/cuda/runtime/cuda_delegate_handle.h>
+#include <executorch/backends/cuda/runtime/cuda_mutable_state.h>
 #include <executorch/backends/cuda/runtime/platform/platform.h>
 #include <executorch/backends/cuda/runtime/shims/memory.h>
 #include <executorch/backends/cuda/runtime/utils.h>
@@ -436,6 +437,10 @@ class ET_EXPERIMENTAL CudaBackend final
           kCudaGraphWarmupSteps);
     }
 
+    // Record whether this AOTI build exposes the constant-management symbols
+    // needed for per-session mutable-buffer rebinding (CUDA V2 multi-session).
+    mutable_state_note_handle(handle);
+
     return (DelegateHandle*)handle; // Return the handle post-processing
   }
 
@@ -539,6 +544,12 @@ class ET_EXPERIMENTAL CudaBackend final
       }
     }
 
+    // CUDA V2 multi-session: if a logical session is active on this thread,
+    // rebind this container's mutable constants (KV/conv/recurrent) to the
+    // session's own GPU buffers before running. No-op for
+    // single-session/legacy.
+    ET_CHECK_OK_OR_RETURN_ERROR(mutable_state_rebind_for_execute(handle));
+
     // ---------------------------------------------------------------
     // CUDA graph REPLAY path — skip all tensor setup and just replay
     // ---------------------------------------------------------------
@@ -826,6 +837,8 @@ class ET_EXPERIMENTAL CudaBackend final
     }
     cuda::CudaDelegateHandle* handle = (cuda::CudaDelegateHandle*)handle_;
 
+    mutable_state_forget_handle(handle);
+
     // The CUDA stream is managed by shared_ptr in the handle.
     // It will be automatically destroyed when the last handle using it
     // is destroyed. Just reset our reference.