ref(upstream): FULL TREE — Deep-Spark xllm (1470) + ds_vllm csrc/models (703)

Replaces cherry-picked upstream_ref with complete source trees.

xllm/ — Iluvatar official C++ inference engine (15MB, 1470 files)
  Complete: kernels → layers → models → runtime → scheduler → api
  Excluded: .git, binary images, third_party submodule checkouts

ds_vllm/ — Iluvatar official vllm fork (8MB, 703 files)
  Included: csrc/ (ALL CUDA kernels), fused_moe/, qwen3_5 model, _custom_ops
  Excluded: tests, benchmarks, docs, examples (not needed for reference)

Critical call chains now fully traceable:
  MoE: moe_topk_softmax_kernels.cuh → ixformer.h → fused_moe.cpp → layer
  GDN: qwen3_gated_delta_net_base.cpp → qwen3_5_gated_delta_net.cpp
  Attention: ixformer.h → xllm_paged_attention → attention.cpp

upstream_ref/xllm/docs/en/features/global_kvcache.md

@@ -0,0 +1,42 @@
+# Global Multi-Level KV Cache
+
+## Background
+In the decoding phase of large language models (LLMs), frequent access to historical KV cache due to autoregressive generation creates a bottleneck in memory bandwidth. As model sizes and context windows expand (e.g., 128K Tokens consuming over 40GB of memory), the pressure on single-device memory increases dramatically. Existing solutions (such as vLLM) exhibit significant limitations in long-context scenarios: prefill time surges, severe memory bandwidth contention during decoding, and the need for excessive resource reservation to meet SLO requirements (TTFT < 2s, TBT < 100ms). This often results in GPU utilization below 40% and difficulties in leveraging cross-server resources. To address this, we propose a distributed global multi-level KV cache management system, adopting a memory-compute integrated architecture to break through single-machine resource constraints.
+
+## Feature Introduction
+The xLLM Global KV Cache feature is primarily implemented through the following three modules:
+-   **etcd**: For cluster service registration, load information synchronization, and global cache state management.
+-   **xLLM Service**: For scheduling requests and managing all compute instances.
+-   **xLLM**: The compute instances handling requests.
+
+The overall architecture is shown in the diagram below:
+![xLLM Global Multi-Level KV Cache](../../assets/globalkvcache_architecture.png)
+
+## Usage Example
+
+### Preparation
+
+#### Install Dependencies
+- **xLLM**: Refer to [Quick Start](../getting_started/quick_start.md)
+- **xLLM Service**: Refer to [PD disaggregation](../getting_started/disagg_pd.md)
+
+### Usage Instructions
+
+1.  **etcd Startup Configuration:**
+    ```bash
+    ./etcd --listen-peer-urls=http://0.0.0.0:10999 --listen-client-urls=http://0.0.0.0:10998
+    ```
+
+2.  **xLLM Service Startup Configuration:**
+    ```bash
+    ./xllm_master_serving --etcd_addr="127.0.0.1:10998" --http_server_port 28888 --rpc_server_port 28889 --tokenizer_path=/path/to/tokenizer_config_dir/
+    ```
+
+3.  **xLLM Startup Configuration:**
+    Add the following gflag parameters when starting xLLM:
+    ```bash
+    --enable_service_routing=true
+    --enable_cache_upload=true
+    # PD separation currently does not support Global KVCache Management 
+    --enable_disagg_pd=false
+    ```