Coverage support for Xen

Coverage support allows you to get coverage information from Xen execution. You can see how many times a line is executed.

Some compilers have specific options that enable the collection of this information. Every basic block in the code will be instrumented by the compiler to compute these statistics. It should not be used in production as it slows down your hypervisor.

GCOV (GCC coverage)

Enable coverage

Test coverage support can be turned on compiling Xen with the CONFIG_COVERAGE option set to y.

Change your .config or run make -C xen menuconfig.

Extract coverage data

To extract data you use a simple utility called xencov. It allows you to do 2 operations:

xencov read extract data
xencov reset reset all coverage counters

Another utility (xencov_split) is used to split extracted data file into files needed by userspace tools.

Split coverage data

Once you extracted data from Xen, it is time to create files which the coverage tools can understand. To do it you need to run xencov_split utility.

The utility just takes an input file and splits the blob into gcc .gcda files in the same directory that you execute the script. As file names are generated relative to the current directory, it could be a good idea to run the script from / on your build machine.

Code for splitting the blob is put in another utility for some reason: * It is simpler to maintain a high level script than a C program; * You don't need to execute on the Xen host so you just need to copy the file to your development box (you usually need development files anyway).

Possible use

This section is just an example on how to use these tools!

This example assumes you compiled Xen from ~/xen-unstable and installed into the host. Consider that if you even recompile Xen you are not able to use blob extracted from xencov!

Ensure the lcov package is installed
From the Xen host machine extract the coverage blob
```
cd /root
xencov read coverage.dat
```
Copy the extracted blob to your dev machine
```
cd ~
scp root@myhost:coverage.dat
```
Extract the coverage information
```
(cd / && xencov_split ~/coverage.dat)
```

Produce coverage html output

cd ~/xen-unstable
rm -rf cov.info cov
geninfo -o cov.info xen
mkdir cov
genhtml -o cov cov.info

See output in a browser
```
firefox cov/index.html
```

LLVM coverage

Enable coverage

Coverage can be enabled using a Kconfig option, from the top-level directory use the following command to display the Kconfig menu:

make -C xen menuconfig clang=y

The code coverage option can be found inside of the "Debugging Options" section. After enabling it just compile Xen as you would normally do:

make xen clang=y

Extract coverage data

LLVM coverage can be extracted from the hypervisor using the xencov tool. The following actions are available:

xencov read extract data
xencov reset reset all coverage counters
xencov read-reset extract data and reset counters at the same time.

Possible use

This section is just an example on how to use these tools!

This example assumes you compiled Xen and copied the xen-syms file from xen/xen-syms into your current directory.

Extract the coverage data from Xen:

xencov read xen.profraw
Convert the data into a profile. Note that you can merge more than one profraw file into a single profdata file.

llvm-profdata merge xen.profraw -o xen.profdata
Generate a HTML report of the code coverage:

llvm-cov show -format=html -output-dir=cov/ xen-syms -instr-profile=xen.profdata
Open cov/index.html with your browser in order to display the profile.