[libc] Remove the 'rpc_reset' routine from the RPC implementation (#66700)

Summary:
This patch removes the `rpc_reset` function. This was previously used to
initialize the RPC client on the device by setting up the pointers to
communicate with the server. The purpose of this was to make it easier
to initialize the device for testing. However, this prevented us from
enforcing an invariant that the buffers are all read-only from the
client side.

The expected way to initialize the server is now to copy it from the
host runtime. This will allow us to maintain that the RPC client is in
the constant address space on the GPU, potentially through inference,
and improving caching behaviour.

Author: jhuber6

libc/config/gpu/entrypoints.txt

@@ -117,7 +117,6 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.time.nanosleep
 
     # gpu/rpc.h entrypoints
-    libc.src.gpu.rpc_reset
     libc.src.gpu.rpc_host_call
 )
 

libc/spec/gpu_ext.td

@@ -5,11 +5,6 @@ def GPUExtensions : StandardSpec<"GPUExtensions"> {
     [], // Types
     [], // Enumerations
     [
-        FunctionSpec<
-            "rpc_reset",
-            RetValSpec<VoidType>,
-            [ArgSpec<UnsignedIntType>, ArgSpec<VoidPtr>]
-        >,
         FunctionSpec<
             "rpc_host_call",
             RetValSpec<VoidType>,

libc/src/__support/RPC/rpc.h

@@ -88,20 +88,13 @@ template <bool Invert, typename Packet> struct Process {
   static constexpr uint64_t NUM_BITS_IN_WORD = sizeof(uint32_t) * 8;
   cpp::Atomic<uint32_t> lock[MAX_PORT_COUNT / NUM_BITS_IN_WORD] = {0};
 
-  /// Initialize the communication channels.
-  LIBC_INLINE void reset(uint32_t port_count, void *buffer) {
-    this->port_count = port_count;
-    this->inbox = reinterpret_cast<cpp::Atomic<uint32_t> *>(
-        advance(buffer, inbox_offset(port_count)));
-    this->outbox = reinterpret_cast<cpp::Atomic<uint32_t> *>(
-        advance(buffer, outbox_offset(port_count)));
-    this->packet =
-        reinterpret_cast<Packet *>(advance(buffer, buffer_offset(port_count)));
-  }
-
-  /// Returns the beginning of the unified buffer. Intended for initializing the
-  /// client after the server has been started.
-  LIBC_INLINE void *get_buffer_start() const { return Invert ? outbox : inbox; }
+  LIBC_INLINE Process(uint32_t port_count, void *buffer)
+      : port_count(port_count), inbox(reinterpret_cast<cpp::Atomic<uint32_t> *>(
+                                    advance(buffer, inbox_offset(port_count)))),
+        outbox(reinterpret_cast<cpp::Atomic<uint32_t> *>(
+            advance(buffer, outbox_offset(port_count)))),
+        packet(reinterpret_cast<Packet *>(
+            advance(buffer, buffer_offset(port_count)))) {}
 
   /// Allocate a memory buffer sufficient to store the following equivalent
   /// representation in memory.
@@ -116,13 +109,13 @@ template <bool Invert, typename Packet> struct Process {
   }
 
   /// Retrieve the inbox state from memory shared between processes.
-  LIBC_INLINE uint32_t load_inbox(uint64_t lane_mask, uint32_t index) {
+  LIBC_INLINE uint32_t load_inbox(uint64_t lane_mask, uint32_t index) const {
     return gpu::broadcast_value(lane_mask,
                                 inbox[index].load(cpp::MemoryOrder::RELAXED));
   }
 
   /// Retrieve the outbox state from memory shared between processes.
-  LIBC_INLINE uint32_t load_outbox(uint64_t lane_mask, uint32_t index) {
+  LIBC_INLINE uint32_t load_outbox(uint64_t lane_mask, uint32_t index) const {
     return gpu::broadcast_value(lane_mask,
                                 outbox[index].load(cpp::MemoryOrder::RELAXED));
   }
@@ -349,13 +342,12 @@ struct Client {
   LIBC_INLINE Client &operator=(const Client &) = delete;
   LIBC_INLINE ~Client() = default;
 
+  LIBC_INLINE Client(uint32_t port_count, void *buffer)
+      : process(port_count, buffer) {}
+
   using Port = rpc::Port<false, Packet<gpu::LANE_SIZE>>;
   template <uint16_t opcode> LIBC_INLINE Port open();
 
-  LIBC_INLINE void reset(uint32_t port_count, void *buffer) {
-    process.reset(port_count, buffer);
-  }
-
 private:
   Process<false, Packet<gpu::LANE_SIZE>> process;
 };
@@ -371,18 +363,13 @@ template <uint32_t lane_size> struct Server {
   LIBC_INLINE Server &operator=(const Server &) = delete;
   LIBC_INLINE ~Server() = default;
 
+  LIBC_INLINE Server(uint32_t port_count, void *buffer)
+      : process(port_count, buffer) {}
+
   using Port = rpc::Port<true, Packet<lane_size>>;
   LIBC_INLINE cpp::optional<Port> try_open();
   LIBC_INLINE Port open();
 
-  LIBC_INLINE void reset(uint32_t port_count, void *buffer) {
-    process.reset(port_count, buffer);
-  }
-
-  LIBC_INLINE void *get_buffer_start() const {
-    return process.get_buffer_start();
-  }
-
   LIBC_INLINE static uint64_t allocation_size(uint32_t port_count) {
     return Process<true, Packet<lane_size>>::allocation_size(port_count);
   }

libc/src/gpu/CMakeLists.txt

@@ -1,14 +1,3 @@
-add_entrypoint_object(
-  rpc_reset
-  SRCS
-    rpc_reset.cpp
-  HDRS
-    rpc_reset.h
-  DEPENDS
-    libc.src.__support.RPC.rpc_client
-    libc.src.__support.GPU.utils
-)
-
 add_entrypoint_object(
   rpc_host_call
   SRCS

libc/src/gpu/rpc_reset.cpp

@@ -44,12 +44,7 @@ static void call_fini_array_callbacks() {
 } // namespace __llvm_libc
 
 extern "C" [[gnu::visibility("protected"), clang::amdgpu_kernel]] void
-_begin(int argc, char **argv, char **env, void *rpc_shared_buffer) {
-  // We need to set up the RPC client first in case any of the constructors
-  // require it.
-  __llvm_libc::rpc::client.reset(__llvm_libc::rpc::MAX_PORT_COUNT,
-                                 rpc_shared_buffer);
-
+_begin(int argc, char **argv, char **env) {
   // We want the fini array callbacks to be run after other atexit
   // callbacks are run. So, we register them before running the init
   // array callbacks as they can potentially register their own atexit

libc/src/gpu/rpc_reset.h

@@ -42,12 +42,7 @@ static void call_fini_array_callbacks() {
 } // namespace __llvm_libc
 
 extern "C" [[gnu::visibility("protected"), clang::nvptx_kernel]] void
-_begin(int argc, char **argv, char **env, void *rpc_shared_buffer) {
-  // We need to set up the RPC client first in case any of the constructors
-  // require it.
-  __llvm_libc::rpc::client.reset(__llvm_libc::rpc::MAX_PORT_COUNT,
-                                 rpc_shared_buffer);
-
+_begin(int argc, char **argv, char **env) {
   // We want the fini array callbacks to be run after other atexit
   // callbacks are run. So, we register them before running the init
   // array callbacks as they can potentially register their own atexit

libc/startup/gpu/amdgpu/start.cpp

@@ -33,13 +33,8 @@ alignas(64) char buffer[alloc_size] = {0};
 
 TEST(LlvmLibcRPCSmoke, SanityCheck) {
 
-  ProcAType ProcA;
-  ProcBType ProcB;
-
-  ProcA.reset(port_count, buffer);
-  ProcB.reset(port_count, buffer);
-
-  EXPECT_EQ(ProcA.get_buffer_start(), ProcB.get_buffer_start());
+  ProcAType ProcA(port_count, buffer);
+  ProcBType ProcB(port_count, buffer);
 
   uint64_t index = 0; // any < port_count
   uint64_t lane_mask = 1;

libc/startup/gpu/nvptx/start.cpp

@@ -34,7 +34,6 @@ struct begin_args_t {
   int argc;
   void *argv;
   void *envp;
-  void *rpc_shared_buffer;
 };
 
 /// The arguments to the '_start' kernel.