Coverage support for Xen

Coverare support allow 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.

Enable coverage

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

Something like:

cd xen
make coverage=y

(or change your .config file).

Extract coverage data

The way GCC and other tools deal with coverage information is to use some files created during build phase (.gcno) and some files produced by executing the program (.gcda). The program in this case is Xen but Xen cannot write files so the way you can use coverage from Xen is extract coverage data from Xen and then split these information into files.

To extract data you use a simple utility called xencov. Mainly xencore allows you to do 3 operations:

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

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
```