ggml-vulkan: pin mmap CPU weights for faster H2D uploads

Export register_host_buffer/unregister via the backend reg so the existing
GGML_CUDA_REGISTER_HOST path in llama-model-loader pins mmap'd expert
weights. Imports the host pages through VK_EXT_external_memory_host, which
populates device->pinned_memory and lets the existing pinned fast path in
ggml_vk_buffer_write_2d_async DMA straight from system RAM instead of
bouncing through the staging buffer + blocking host memcpy.

Assisted-by: opencode

ggml/src/ggml-vulkan/ggml-vulkan.cpp

@@ -17917,11 +17917,74 @@ static ggml_backend_dev_t ggml_backend_vk_reg_get_device(ggml_backend_reg_t reg,
     return devices[device];
 }
 
+// Import an mmap-backed host region as a Vulkan pinned buffer via
+// VK_EXT_external_memory_host so H2D uploads DMA straight from system RAM
+// instead of bouncing through the staging buffer + host memcpy. Mirrors the
+// GGML_CUDA_REGISTER_HOST path; populates device->pinned_memory, which the
+// existing pinned fast path in ggml_vk_buffer_write_2d_async looks up.
+static bool ggml_backend_vk_register_host_buffer(void * buffer, size_t size) {
+    if (getenv("GGML_CUDA_REGISTER_HOST") == nullptr && getenv("GGML_VK_REGISTER_HOST") == nullptr) {
+        return false;
+    }
+    if (size == 0) {
+        return false;
+    }
+
+    bool success = false;
+    for (size_t i = 0; i < GGML_VK_MAX_DEVICES; i++) {
+        vk_device& device = vk_instance.devices[i];
+        if (!device || !device->external_memory_host) {
+            continue;
+        }
+
+        vk_buffer buf = ggml_vk_buffer_from_host_ptr(device, buffer, size);
+        if (!buf || !buf->buffer) {
+            continue;
+        }
+
+        {
+            std::lock_guard<std::shared_mutex> guard(device->pinned_memory_mutex);
+            device->pinned_memory.emplace_back(buffer, size, buf);
+        }
+        success = true;
+    }
+    return success;
+}
+
+static void ggml_backend_vk_unregister_host_buffer(void * buffer) {
+    for (size_t i = 0; i < GGML_VK_MAX_DEVICES; i++) {
+        vk_device& device = vk_instance.devices[i];
+        if (!device) {
+            continue;
+        }
+        std::lock_guard<std::shared_mutex> guard(device->pinned_memory_mutex);
+        for (auto it = device->pinned_memory.begin(); it != device->pinned_memory.end(); ++it) {
+            if (std::get<0>(*it) == buffer) {
+                vk_buffer buf = std::get<2>(*it);
+                device->pinned_memory.erase(it);
+                ggml_vk_destroy_buffer(buf);
+                break;
+            }
+        }
+    }
+}
+
+static void * ggml_backend_vk_reg_get_proc_address(ggml_backend_reg_t reg, const char * name) {
+    GGML_UNUSED(reg);
+    if (strcmp(name, "ggml_backend_register_host_buffer") == 0) {
+        return (void *) ggml_backend_vk_register_host_buffer;
+    }
+    if (strcmp(name, "ggml_backend_unregister_host_buffer") == 0) {
+        return (void *) ggml_backend_vk_unregister_host_buffer;
+    }
+    return nullptr;
+}
+
 static const struct ggml_backend_reg_i ggml_backend_vk_reg_i = {
     /* .get_name         = */ ggml_backend_vk_reg_get_name,
     /* .get_device_count = */ ggml_backend_vk_reg_get_device_count,
     /* .get_device       = */ ggml_backend_vk_reg_get_device,
-    /* .get_proc_address = */ NULL,
+    /* .get_proc_address = */ ggml_backend_vk_reg_get_proc_address,
 };
 
 ggml_backend_reg_t ggml_backend_vk_reg() {