[DRAFT: DONT COMMIT] Enable zero-copy subbuffers across all alloc types for iGPUs

#### Details:
Description of the issue (symptom, root-cause, how it was resolved)
- Eliminates redundant weight copies during model loading for integrated GPUs by binding
  mmap'd cache file memory directly to GPU without intermediate usm_device allocation
- Reduces memory footprint during inference by reusing host memory as GPU-accessible buffers
- Zero-copy enabled when: iGPU, XE2+ architecture, USM host support available
- Falls back gracefully to copy-based path when zero-copy requirements not met

Implementation changes:
- Add ocl_engine::supports_hostbuffer() to validate zero-copy capability
  (checks for integrated GPU, XE2+ arch, USM host allocation support)
- Add ocl_engine::create_hostbuffer_impl() to bind host memory with CL_MEM_USE_HOST_PTR
- Simplify zero-copy detection in data.hpp load path (remove redundant capability checks)
- Use ib.seek_current_ptr() instead of dummy reads for padding alignment
- Remove is_mmap_tensor_4K_aligned() helpers

Changed files:
src/plugins/intel_gpu/include/intel_gpu/graph/serialization/binary_buffer.hpp
src/plugins/intel_gpu/include/intel_gpu/primitives/data.hpp
src/plugins/intel_gpu/include/intel_gpu/runtime/device_info.hpp
src/plugins/intel_gpu/src/graph/program.cpp
src/plugins/intel_gpu/src/runtime/ocl/ocl_device.cpp
src/plugins/intel_gpu/src/runtime/ocl/ocl_engine.cpp
src/plugins/intel_gpu/tests/unit/test_cases/cache_serialization_test.cpp

Reproduction step and snapshot (if applicable. Do not attach for customer model)
NA

Problematic graph
NA

#### Checklist
- [x] Is it a proper fix? Yes (memory optimization, not a workaround)
- [x] Did you include test case for this fix, if necessary? NA
- [x] Did you review existing test that can be extended to cover this scenario? NA

#### Tickets:
 - CVS-176160

#### AI Assistance:
 - AI assistance used: yes
 - AI was used for code implementation, naming suggestions, and comment refinement.

Author: Prithviraj-R

src/plugins/intel_gpu/include/intel_gpu/graph/serialization/binary_buffer.hpp

@@ -14,13 +14,6 @@
 
 namespace cldnn {
 
-/// @brief Page size alignment constant for cache blob serialization.
-/// This value (4KB) matches the typical OS page size on Windows, Linux, and macOS.
-inline constexpr uint32_t CACHE_PAGE_SIZE = 4096;
-
-/// @brief Alignment requirement for individual weight buffers within cache blobs.
-/// This alignment (128 bytes) ensures optimal GPU memory access patterns for weight data.
-inline constexpr uint32_t CACHE_SUB_BUFFER_ALIGNMENT = 128;
 struct memory;
 
 class BinaryOutputBuffer : public OutputBuffer<BinaryOutputBuffer> {
@@ -61,20 +54,12 @@ class BinaryOutputBuffer : public OutputBuffer<BinaryOutputBuffer> {
     size_t get_offset() const {
         return _offset;
     }
-    bool is_offset_page_aligned() const {
-        return (get_offset() % CACHE_PAGE_SIZE == 0);
-    }
 
-    size_t get_bytes_to_page_boundary() const {
-        return CACHE_PAGE_SIZE - (get_offset() % CACHE_PAGE_SIZE);
+    bool is_offset_aligned(const size_t alignment) const {
+        return (get_offset() % alignment == 0);
     }
-
-    bool is_offset_sub_buffer_aligned() const {
-        return (get_offset() % CACHE_SUB_BUFFER_ALIGNMENT == 0);
-    }
-
-    size_t get_bytes_to_sub_buffer_boundary() const {
-        return CACHE_SUB_BUFFER_ALIGNMENT - (get_offset() % CACHE_SUB_BUFFER_ALIGNMENT);
+    size_t get_bytes_to_aligned_boundary(const size_t alignment) const {
+        return alignment - (get_offset() % alignment);
     }
 
 private:
@@ -117,13 +102,6 @@ class BinaryInputBuffer : public InputBuffer<BinaryInputBuffer> {
     std::streambuf* get_streambuf() const {
         return _stream.rdbuf();
     }
-    bool has_mmap_tensor() const {
-        return _tensor_base_ptr != nullptr;
-    }
-
-    bool is_mmap_tensor_4K_aligned() const {
-        return has_mmap_tensor() && (reinterpret_cast<std::uintptr_t>(_tensor_base_ptr) % CACHE_PAGE_SIZE == 0);
-    }
 
     const size_t* get_mmap_tensor() const {
         return _tensor_base_ptr;
@@ -142,28 +120,22 @@ class BinaryInputBuffer : public InputBuffer<BinaryInputBuffer> {
     size_t get_offset() const {
         return _offset;
     }
-
+    bool has_mmap_tensor() const {
+        return _tensor_base_ptr != nullptr;
+    }
     void seek_current_ptr(std::streamsize size) {
         // Get current stream position
         std::streampos current_pos = _stream.tellg();
         // Advance stream position
         _stream.seekg(current_pos + static_cast<std::streampos>(size));
         _offset += size;
     }
-    bool is_offset_page_aligned() const {
-        return (get_offset() % CACHE_PAGE_SIZE == 0);
-    }
-
-    size_t get_bytes_to_page_boundary() const {
-        return CACHE_PAGE_SIZE - (get_offset() % CACHE_PAGE_SIZE);
-    }
 
-    bool is_offset_sub_buffer_aligned() const {
-        return (get_offset() % CACHE_SUB_BUFFER_ALIGNMENT == 0);
+    bool is_offset_aligned(const size_t alignment) const {
+        return (get_offset() % alignment == 0);
     }
-
-    size_t get_bytes_to_sub_buffer_boundary() const {
-        return CACHE_SUB_BUFFER_ALIGNMENT - (get_offset() % CACHE_SUB_BUFFER_ALIGNMENT);
+    size_t get_bytes_to_aligned_boundary(const size_t alignment) const {
+        return alignment - (get_offset() % alignment);
     }
     void setKernelImplParams(void* impl_params) {
         _impl_params = impl_params;

src/plugins/intel_gpu/include/intel_gpu/primitives/data.hpp

@@ -394,11 +394,12 @@ struct data : public primitive_base<data> {
 
         bool do_weightless_caching = cache_info->save(ob, data_size);
         if (!do_weightless_caching) {
+            auto sub_buffer_base_alignment = mem->get_engine()->get_device_info().sub_buffer_base_alignment;
+            if (!ob.is_encrypted() && !ob.is_offset_aligned(sub_buffer_base_alignment)) {
+                std::vector<uint8_t> pad(ob.get_bytes_to_aligned_boundary(sub_buffer_base_alignment), 0);
+                ob << make_data(pad.data(), pad.size());
+            }
             if (is_alloc_host_accessible(_allocation_type)) {
-                if (!ob.is_encrypted() && !ob.is_offset_sub_buffer_aligned()) {
-                    std::vector<uint8_t> pad(ob.get_bytes_to_sub_buffer_boundary(), 0);
-                    ob << make_data(pad.data(), pad.size());
-                }
                 ob << make_data(mem->buffer_ptr(), data_size);
             } else {
                 std::vector<uint8_t> _buf;
@@ -430,29 +431,24 @@ struct data : public primitive_base<data> {
 
         bool weightless_caching = false;
         ib >> weightless_caching;
-
-        bool enable_zero_copy_mode = ib.is_mmap_tensor_4K_aligned() && ib.get_engine().get_device_info().arch >= gpu_arch::xe2 &&
-                                     ib.get_engine().get_device_info().dev_type == device_type::integrated_gpu &&
-                                     _allocation_type == allocation_type::usm_host && !weightless_caching &&
-                                     model_tensor_base != nullptr;
-        if (!enable_zero_copy_mode) {
+        bool zero_copy_mode = !weightless_caching && model_tensor_base != nullptr && ib.has_mmap_tensor();
+        if (!zero_copy_mode) {
             mem = ib.get_engine().allocate_memory(output_layout, _allocation_type, false);
         }
 
         bool is_weightless_caching = cache_info->load(ib, mem, weights_memory, weightless_caching);
 
         if (!is_weightless_caching) {
-            if (is_alloc_host_accessible(_allocation_type)) {
-                if (!ib.is_encrypted() && !ib.is_offset_sub_buffer_aligned()) {
-                    std::vector<uint8_t> pad(ib.get_bytes_to_sub_buffer_boundary(), 0);
-                    ib >> make_data(pad.data(), pad.size());
-                }
-                if (enable_zero_copy_mode) {
-                    mem = ib.get_engine().create_subbuffer(*model_tensor_base, output_layout, ib.get_offset());
-                    ib.seek_current_ptr(data_size);
-                } else {
-                    ib >> make_data(std::move(mem->buffer_ptr()), data_size);
-                }
+            auto sub_buffer_base_alignment = ib.get_engine().get_device_info().sub_buffer_base_alignment;
+            if (!ib.is_encrypted() && !ib.is_offset_aligned(sub_buffer_base_alignment)) {
+                ib.seek_current_ptr(ib.get_bytes_to_aligned_boundary(sub_buffer_base_alignment));
+            }
+            // Zero-copy: create subbuffer referencing mmap'd cache without host-to-device transfer
+            if (zero_copy_mode) {
+                mem = ib.get_engine().create_subbuffer(*model_tensor_base, output_layout, ib.get_offset());
+                ib.seek_current_ptr(data_size);
+            } else if (is_alloc_host_accessible(_allocation_type)) {
+                ib >> make_data(std::move(mem->buffer_ptr()), data_size);
             } else {
                 const size_t DATA_BLOCK_SIZE = 4 * 1024 * 1024;
                 auto& eng = ib.get_engine();

src/plugins/intel_gpu/include/intel_gpu/runtime/device_info.hpp

@@ -145,7 +145,7 @@ struct device_info {
     uint32_t num_ccs;                           ///< Number of compute command streamers
     uint32_t sub_device_idx;                    ///< Index of sub-device
     int32_t cacheline_size;
-
+    uint32_t sub_buffer_base_alignment;
     pci_bus_info pci_info;                      ///< PCI bus information for the device
 
     uint64_t timer_resolution;                  ///< [ZE] Resolution of device timer used for profiling in cycles/sec

src/plugins/intel_gpu/src/graph/program.cpp

@@ -1985,9 +1985,10 @@ void program::save(cldnn::BinaryOutputBuffer& ob) const {
         ob << state_initializer.first;
         ob << state_initializer.second;
     }
-
-    if (!ob.is_encrypted() && !ob.is_offset_page_aligned()) {
-        std::vector<uint8_t> pad(ob.get_bytes_to_page_boundary(), 0);
+    
+    auto minimal_alignment = get_engine().get_device_info().cacheline_size;
+    if (!ob.is_encrypted() && !ob.is_offset_aligned(minimal_alignment)) {
+        std::vector<uint8_t> pad(ob.get_bytes_to_aligned_boundary(minimal_alignment), 0);
         ob << make_data(pad.data(), pad.size());
     }
 }
@@ -2016,10 +2017,17 @@ void program::load(cldnn::BinaryInputBuffer& ib,
             OPENVINO_THROW("Weights path or model is required for cache mode OPTIMIZE_SIZE");
         }
     }
-
-    const bool can_use_mmap_zero_copy = ib.is_mmap_tensor_4K_aligned() && _engine.get_device_info().arch >= gpu_arch::xe2 &&
+    const bool can_use_mmap_zero_copy = ib.has_mmap_tensor() && _engine.get_device_info().arch >= gpu_arch::xe2 &&
                                         _engine.get_device_info().dev_type == device_type::integrated_gpu && !_config.get_enable_weightless();
     memory_ptr model_tensor_base_ptr = nullptr;
+    // Checks if zero-copy host memory binding is supported.
+    // Requires: integrated GPU, XE2+ architecture, USM host allocation
+    const auto supports_hostbuffer = [&](const cldnn::engine& eng) {
+        const auto& info = eng.get_device_info();
+        return info.dev_type == cldnn::device_type::integrated_gpu && info.arch >= cldnn::gpu_arch::xe2 &&
+               eng.supports_allocation(cldnn::allocation_type::usm_host) && ib.has_mmap_tensor();
+    };
+
     if (can_use_mmap_zero_copy) {
         model_tensor_base_ptr =
             ib.get_engine().create_hostbuffer(ib.get_mmap_tensor(),
@@ -2214,10 +2222,9 @@ void program::load(cldnn::BinaryInputBuffer& ib,
         state_initializers[variable_id] = initializers;
     }
 
-    // At the end of load
-    if (!ib.is_encrypted() && !ib.is_offset_page_aligned()) {
-        std::vector<uint8_t> pad(ib.get_bytes_to_page_boundary(), 0);
-        ib >> make_data(pad.data(), pad.size());
+    auto minimal_alignment = get_engine().get_device_info().cacheline_size;
+    if (!ib.is_encrypted() && !ib.is_offset_aligned(minimal_alignment)) {
+        ib.seek_current_ptr(ib.get_bytes_to_aligned_boundary(minimal_alignment));
     }
 }
 

src/plugins/intel_gpu/src/runtime/ocl/ocl_device.cpp

@@ -209,6 +209,9 @@ device_info init_device_info(const cl::Device& device, const cl::Context& contex
     info.sub_device_idx = std::numeric_limits<uint32_t>::max();
 
     info.cacheline_size = device.getInfo<CL_DEVICE_GLOBAL_MEM_CACHELINE_SIZE>();
+    // Alignment requirement (in bits) for sub-buffer offsets, converting to bytes and ensuring at least 1 byte alignment.
+    auto bits = device.getInfo<CL_DEVICE_MEM_BASE_ADDR_ALIGN>(); 
+    info.sub_buffer_base_alignment = std::max<uint32_t>(1, bits / 8);
     info.execution_units_count = device.getInfo<CL_DEVICE_MAX_COMPUTE_UNITS>();
 
     info.gpu_frequency = static_cast<uint32_t>(device.getInfo<CL_DEVICE_MAX_CLOCK_FREQUENCY>());

src/plugins/intel_gpu/src/runtime/ocl/ocl_engine.cpp

@@ -4,7 +4,6 @@
 
 #include "ocl_engine.hpp"
 #include "intel_gpu/runtime/utils.hpp"
-#include "intel_gpu/graph/serialization/binary_buffer.hpp"  // For CACHE_PAGE_SIZE
 #include "openvino/runtime/intel_gpu/remote_properties.hpp"
 
 #include "ocl_kernel.hpp"

src/plugins/intel_gpu/tests/unit/test_cases/cache_serialization_test.cpp

@@ -330,8 +330,10 @@ membuf make_oversized_data_blob(const layout& output_layout, size_t malicious_da
     ob << weightless_caching;
 
     // Mirror the reader's alignment padding so stream offsets stay in sync.
-    if (!ob.is_encrypted() && !ob.is_offset_sub_buffer_aligned()) {
-        std::vector<uint8_t> pad(ob.get_bytes_to_sub_buffer_boundary(), 0);
+    auto& engine = get_test_engine();
+    auto sub_buffer_base_alignment = engine.get_device_info().sub_buffer_base_alignment;
+    if (!ob.is_encrypted() && !ob.is_offset_aligned(sub_buffer_base_alignment)) {
+        std::vector<uint8_t> pad(ob.get_bytes_to_aligned_boundary(sub_buffer_base_alignment), 0);
         ob << make_data(pad.data(), pad.size());
     }