# Porting Graphene PAL¶

Graphene adopts a similar architecture to the Drawbridge Library OS, which runs a generic library OS on top of a Platform Adaptation Layer (PAL) to maximize platform compatibility. In this architecture, the library OS can be easily ported to a new host by implementing only the PAL for this new host.

To port Graphene to a new host platform, the only effort required is reimplementing the PAL on the desired host platform. Most of the implementation should be as simple as translating the PAL API to the native system interface of the host. The implemented PAL must support PAL host ABI.

In fact, even in the PAL source code, we expect part of the code to be host-generic. To make porting Graphene easier, we deliberately separate the source code of PAL into three parts:

• Pal/lib: All the library APIs used internally by PAL.
• Pal/src: Host-generic implementation.
• Pal/src/host/host name: Host-specific implementation.

To port Graphene to a new host, we suggest starting with a clone of Pal/src/host/Skeleton. This directory contains the skeleton code of all functions that need to be implemented as part of a fully compatible PAL. Although we have tried our best to isolate any host-specific code in each host directory, we do not guarantee that the necessary changes are only limited to these directories. That is, you may have to modify other parts of the source code (especially the Makefile scripts) to complete your implementation.

Below are the steps to port Graphene PAL to a new host platform.

## 1. Fix compilation issues¶

For the first step to port PAL, you want to be able to build PAL as an executable on the target host. After cloning a host-specific directory, first modify Makefile.am to adjust compilation rules such as CC, CFLAGS, LDFLAGS, AS and ASFLAGS. You will also have to define the name of the loader as target pal in Makefile.am.

PAL needs to run on the target host like a regular executable. To run Graphene, PAL must initialize the proper environments and load the applications as well as the library OS in the form of Linux ELF binaries. To start the implemention of PAL loader, we suggest you begin with the following APIs in your host-specific directory:

1. db_main.c: This file must contain the entry function of your loader (the main() function) and APIs to retrieve host-specific information. The definitions of the APIs are as follows:
• _DkGetAllocationAlignment (required): Return the allocation alignment (granularity) of the target platform. Some platforms have different allocation alignments rather than the usual page-size alignment.
• _DkGetAvailableUserAddressRange (required): PAL must provide a user address range that applications can use. None of these addresses should be used by PAL internally.
• _DkGetProcessId (required): Return a unique process ID for each process.
• _DkGetCPUInfo (optional): Retrieve CPU information, such as vendor ID, model name.

The entry function in db_main.c must eventually call the generic entry point pal_main(). The definition of pal_main() is:

void pal_main(PAL_NUM instance_id, const char * exec_uri, PAL_HANDLE parent_process, PAL_HANDLE first_thread, PAL_STR * arguments, PAL_STR * environments)

Main initialization function.

This function must be called by the host-specific loader.

Parameters
• instance_id: current instance id
• exec_uri: executable URI
• parent_process: parent process if it’s a child
• first_thread: first thread handle
• arguments: application arguments
• environments: environment variables

1. pal_host.h: This file needs to define the member of PAL_HANDLE for handles of files, devices, pipes, sockets, threads, processes, etc.
2. db_files.c: To implement a basic loader, you have to specify how to open, read, and map an executable file. At least file_open, file_read, file_map, file_attrquery, file_attrquerybyhdl must be implemented to load a basic HelloWorld program.
3. db_memory.c: The same as db_files.c, this file also contain APIs essential to PAL loader. At least _DkCheckMemoryMappable, _DkVirtualMemoryAlloc, _DkVirtualMemoryFree, _DkVirtualMemoryProtect must be implemented.
4. db_rtld.c: This file must handle how symbols are resolved against the PAL loader itself, to discover the entry address of the host ABI. If the PAL loader is a Linux ELF binary, you may simply add a link_map to the g_loaded_maps list. Otherwise, you need to implement resolve_rtld function to return addresses of the host ABI by names.

You may implement the optional _DkDebugMapAdd and _DkDebugMapRemove to use a host-specific debugger such as GDB to debug applications in Graphene.

In Pal/test, we provide a test program that can run without the library OS and directly use the PAL host ABI. If you can successfully run a HelloWorld program, congratulations, you have a working PAL loader.

## 4. Implementing PAL host ABI¶

Now it is time to complete the whole implementation of the PAL host ABI. Once you have finished implementation, use the regression tests to confirm whether your implementation is compatible with the PAL Host ABI. To run the regression tests, run the following steps:

cd Pal/regression
make regression


## 5. Running application with library OS¶

With a completely implemented PAL, you should be able to run any applications that are currently supported by Graphene on your new platform. Please be aware you should not try to build any application binaries on your target host. On the contrary, you should build them on a Linux host and ship them to your target host.