Unikraft Hello World Application with Coverage Support (gcov)

This builds the Unikraft helloworld image with coverage support used by gcov. The actual support is provided in gcc via the -fprofile-arcs option. Coverage support can be used for profile-guided optimization (PGO). This is why the usual way to compile and link a program under test is via the -fprofile-generate option; it includes the -fprofile-arcs option.

Building and running the application can be done either using kraft or with the manual approach.

Dependencies

Apart from Unikraft, the following libraries / dependencies are required:

• lib-pthread-embedded
• lib-libunwind
• lib-compiler-rt
• lib-libcxxabi
• lib-libcxx
• lib-newlib
• lib-gcc

For lib-gcc, use the UPB fork, the gcov branch. Similarly, for Unikraft, use the UPB fork, the gcov branch. For the other libraries, use the staging branch.

Preparation

The profiling information is generated as a .gcda file. The application requires filesystem support to generate / use the profiling information file. The application will be configured to enable the use of the 9pfs filesystem. 9fps is a network filesystem that maps a local folder as the root directory in the application running in a KVM virtual machine. We use the local folder fs0 for this. Create it using:

mkdir fs0

Build using kraft

The easiest way to build and run the Unikraft image is by using kraft.

The custom UPB forks for Unikraft and lib-gcc are required. Create a custom folder structure for Unikraft and required dependencies:

• Clone the UPB fork for Unikraft. The clone name has to be unikraft (the default name). Use the gcov branch.
• Create a folder named libs/. The unikraft clone and the libs/ folder are placed in the same folder.
• Clone the library repositories in the libs/ folder. Be sure to name them according to kraft's naming scheme: pthread-embedded, libunwind, compiler-rt, libcxxabi, libcxx, newlib, gcc. Use the gcov branch for lib-gcc library. Use the staging branch for the other libraries.

For kraft, use the UK_WORKDIR kraft environment variable to point to the folder storing the unikraft clone and the libs/ folder. In the commands below, replace /path/to/folder with the actual path to this folder.

First configure the application:

UK_WORKDIR=/path/to/folder kraft configure -m x86_64 -p kvm

Then build the application:

UK_WORKDIR=/path/to/folder kraft build

The first building of the application will take some time, as library files are downloaded, unpacked and built. The resulting KVM image file is build/app-helloworld-gcov_kvm-x86_64.

Build using the manual approach

The manual approach gives more control on the configuration and build process.

Dependencies have to be included in the order in the Makefile. Update the Makefile variables (UK_ROOT, UK_LIBS, LIBS) according to your setup.

Configure the application via the configuration screen:

make menuconfig

In the configuration menu, select Library Configuration -> vfscore: Configuration -> Default root device and type fs0. The rest of the configuration is loaded automatically from the Config.uk file.

Build the application:

make

The first building of the application will take some time, as library files are downloaded, unpacked and built. The resulting KVM image is build/app-helloworld-gcov_kvm-x86_64. The image name may be updated in the configuration screen (make menuconfig), using the Image name option.

Building for optimization

An application with coverage support can be built in two ways:

1. to generate the profiling information: the -fprofile-generate option
2. to use the generated information for optimization, i.e. the actual profile-guided optimization (PGO): the -fprofile-use option

By default, the application is built to generate the profile, i.e. using the -fprofile-generate option.

If building the application for optimization, i.e. using the -fprofile-use option, its configuration needs to be updated.

If using kraft, update the kraft.yaml. Comment out the CONFIG_OPTIMIZE_PGO_GENERATE=y line and uncomment the CONFIG_OPTIMIZE_PGO_USE=y line.

If using the manual approach, enter the configuration screen (make menuconfig) and select Build options -> PGO - Profile-guided optimization -> PGO Options -> profile-use.

Then rebuild the image using kraft or the manual approach.

Run

If using kraft, run the application with the command:

UK_WORKDIR=/path/to/folder kraft run -p kvm

Replace /path/to/folder with the path to the folder storing the unikraft clone and the libs/ folder.

If using the manual approach, use the qemu-guest script in kraft:

qemu-guest -k build/app-helloworld-gcov_kvm-x86_64 -e fs0

Note that kraft also uses this script as part of its kraft run command. So the qemu-guest command above can also be used for the kraft build.

Running the application, either using kraft or the qemu-guest script, requires root privileges.

After running the profile-generation image, the output file will be generated in fs0/gcov_profiling/main.gcda. The file will be used by the profile-use phase after re-building the application for optimization. The file is created with root privileges. Modifying or removing it requires root privileges.