sycl : support dev2dev memcpy by DEV2DEV_MEMCPY_FORWARD (#26234)

Co-authored-by: Neo Zhang Jianyu <jianyu.zhang@intel.com>

ggml/src/ggml-sycl/common.hpp

@@ -133,6 +133,7 @@ enum ggml_sycl_backend_gpu_mode {
 enum ggml_sycl_dev2dev_memcpy_mode {
   DEV2DEV_MEMCPY_SYCL = 0,
   DEV2DEV_MEMCPY_L0 = 1,
+  DEV2DEV_MEMCPY_FORWARD = 2
 };
 
 static_assert(sizeof(sycl::half) == sizeof(ggml_fp16_t), "wrong fp16 size");

ggml/src/ggml-sycl/ggml-sycl.cpp

@@ -274,6 +274,8 @@ static const char* dev2dev_int2str(int dev2dev) {
         return "SYCL API";
     } else if (dev2dev == DEV2DEV_MEMCPY_L0) {
         return "Level Zero API";
+    } else if (dev2dev == DEV2DEV_MEMCPY_FORWARD) {
+        return "Host Forward";
     } else {
         return "Unknown";
     }
@@ -684,7 +686,11 @@ static void dev2dev_memcpy(int device_dst, sycl::queue &q_dst, int device_src, s
     }
 
     // Host-staged copy
-    GGML_SYCL_DEBUG("[SYCL] dev2dev memcpy by host forward\n");
+    if(g_ggml_sycl_dev2dev_memcpy == DEV2DEV_MEMCPY_FORWARD) {
+        GGML_SYCL_DEBUG("[SYCL] dev2dev memcpy by host forward for setting GGML_SYCL_DEV2DEV_MEMCPY=2\n");
+    } else {
+        GGML_SYCL_DEBUG("[SYCL] dev2dev memcpy by host forward for SYCL/L0 fallback\n");
+    }
     char *host_buf = (char *)malloc(size);
     q_src.memcpy(host_buf, (const char *)ptr_src, size).wait();
     q_dst.memcpy((char *)ptr_dst, host_buf, size).wait();