Test Suite - KVM

From Libreswan
Jump to navigation Jump to search

KVM Test framework

Libreswan's test framework can be run using KVM guests, and the kvm scripts. It is strongly recommended to run the test suite on a host machine that has a CPU wth virtualisation instructions.

To access files on the host file system:

  • Linux guests (Fedora) use the PLAN9 filesystem (9p)
  • BSD guests (FreeBSD, NetBSD, OpenBSD) use NFS via the NAT interface

For an overview of the tests see Test_Suite

Preparing the host machine

Check Virtualization is enabled in the BIOS

Virtualization needs to be enabled by the BIOS during boot.

 grep -e vmx -e svm /proc/cpuinfo

Add yourself to sudo

Some of the test scrips need to be run as root. The test environment assumes this can be done using sudo without a password vis:

sudo pwd

XXX: Surely qemu can be driven without root?

This is setup by adding an entry under /etc/sudoers.d/ specifying that your account does not need a password to become root:

echo "$(id -u -n) ALL=(ALL) NOPASSWD: ALL" | sudo dd of=/etc/sudoers.d/$(id -u -n)

Fight SELinux

SELinux blocks some actions that we need. We have not created any SELinux rules to avoid this. The options are:

  • set SELinux to permissive (recommended)
sudo sed --in-place=.ORIG -e 's/^SELINUX=.*/SELINUX=permissive/' /etc/selinux/config
sudo setenforce Permissive
  • disable SELinux
sudo sed --in-place=.ORIG -e 's/^SELINUX=.*/SELINUX=disabled/' /etc/selinux/config
sudo reboot
  • (experimental) label source tree for SELinux

The source tree on the host is shared with the virtual machines. SELinux considers this a bug unless the tree is labelled with type svirt_image_t.

sudo dnf install policycoreutils-python-utils
sudo semanage fcontext -a -t svirt_image_t "$(pwd)"'(/.*)?'
sudo restorecon -vR /home/build/libreswan

There may be other things that SELinux objects to.

Check that the host has enough entropy

As a rough guide run:

while true ; do cat /proc/sys/kernel/random/entropy_avail ; sleep 3 ; done

it should have values in the hundrets if not thousands. If it is in the units or tens then see Entropy matters

Install Dependencies

Why Fedora Mint (debian)
Basics sudo dnf install -y make git gitk patch sudo apt-get install -y make make-doc git gitk
Python sudo dnf install -y python3-pexpect sudo apt-get install python3-pexpect
Virtulization sudo dnf install -y qemu virt-install libvirt-daemon-kvm libvirt-daemon-qemu sudo apt install -y qemu virtinst libvirt-clients libvirt-daemon libvirt-daemon-system libvirt-daemon-driver-qemu libosinfo-query qemu-system-x86?
Boot CDs sudo dnf install -y dvd+rw-tools sudo apt-get install -y dvd+rw-tools
Web Pages sudo dnf install -y jq nodejs-typescript sudo apt-get install -y jq node-typescript
Web Server (optional) sudo dnf install -y httpd sudo apt-get install -y ????
NFS sudo dnf install -y nfs-utils # ??? sudo apt-get install -y nfs-kernel-server rpcbind
Broken makefiles sudo dnf install -y nss-devel # make file invokes pkg-config nss

Enable libvirt

If you're switching from the old libvirtd see https://libvirt.org/daemons.html#switching-to-modular-daemons for how to shut down the old daemons.

Start the "collection of modular daemons that replace functionality previously provided by the monolithic libvirtd daemon":

for drv in qemu network nodedev nwfilter secret storage interface
   sudo systemctl unmask virt${drv}d.service
   sudo systemctl unmask virt${drv}d{,-ro,-admin}.socket
   sudo systemctl enable virt${drv}d.service
   sudo systemctl enable virt${drv}d{,-ro,-admin}.socket
for drv in qemu network nodedev nwfilter secret storage
   sudo systemctl start virt${drv}d{,-ro,-admin}.socket

There should be no errors and warnings.

Stop libvirt daemons shutting down

By default the libvirt daemons timeout and shutdown after 120 seconds (surely systemd will restart them!). It turns out this hasn't worked so well:

systemd doesn't restart the daemon
the restart is painfully slow with lots of networks which causes the timeout

Disabling the timeout and just leaving the daemons running seems to help. Add the following:

 echo VIRTNETWORKD_ARGS= > /etc/sysconfig/virtnetworkd
 echo VIRTQEMUD_ARGS= > /etc/sysconfig/virtqemud
 echo VIRTSTORAGED_ARGS= > /etc/sysconfig/virtstoraged

the standard libvirt systemd config files read these settings using EnvironmentFile=

Add yourself to the KVM/QEMU group

You need to add yourself to the group that QEMU/KVM uses when writing to /var/lib/libvirt/qemu. On Fedora it is 'qemu', and on Debian it is 'kvm'. Something like:

sudo usermod -a -G $(stat --format %G /var/lib/libvirt/qemu) $(id -u -n)

After this you will will need to re-login (or run sudo su - $(id -u -n)

Make certain that root can access the build

The path to your build needs to be accessible (executable) by root, assuming things are under home:

chmod a+x $HOME

Fix /var/lib/libvirt/qemu

sudo chmod g+w /var/lib/libvirt/qemu

Arguably we should run libvirt as a normal user instead.

Create /etc/modules-load.d/virtio.conf (obsolete since 2022 at least)

Several virtio modules need to be loaded into the host's kernel. This could be done by modprobe ahead of running any virtual machines but it is easier to install them whenever the host boots. This is arranged by listing the modules in a file within /etc/modules-load.d. The host must be rebooted for this to take effect.

sudo dd <<EOF of=/etc/modules-load.d/virtio.conf

As of Fedora 28, several of these modules are built into the kernel and will not show up in /proc/modules (virtio, virtio_rng, virtio_pci, virtio_ring).

Set up a Web Server (optional)

If the machine is to run nightly test runs then it can be set up as a web server. See the nightly test results for an example.

See above for dependencies. See below for how to configure libreswan.

To set up the server:

sudo mkdir /var/www/html/results/
sudo chown $(id -un) /var/www/html/results/
sudo chmod 755 /var/www/html/results/
sudo sh -c 'echo "AddType text/plain .diff" >/etc/httpd/conf.d/diff.conf'

to run the web server until the next reboot:

sudo firewall-cmd --add-service=http
sudo systemctl start httpd

to make the web server permanent:

sudo systemctl enable httpd
sudo firewall-cmd --add-service=http --permanent

If you want it to be the main page of the website, you can create the file /var/www/html/index.html containing:

cat <<EOF
 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
     <meta http-equiv="REFRESH" content="0;url=/results/">


On Debian slack based systems (i.e., Linux Mint 20.3), the default python is too old. Fortunately python 3.9 is also available vis:

sudo apt-get install python3.9

in addition, the make variable KVM_PYTHON will need to be added to Makefile.inc.local, see below.



Download and configure libreswan

Fetch Libreswan

The libreswan source tree includes all the components that are used on the host and inside the test VMs. To get the latest source code using git:

git clone https://github.com/libreswan/libreswan
cd libreswan

Developers can use Makefile.inc.local to override default build setttings. Create the file:

touch Makefile.inc.local

(packaging systems should not use this, and instead explicitly pass the make variables to the make command)

(optional) Override python

See above. If pthon isn't new enough (for instance on Debian) set KVM_PYTHON in Makefile.inc.local vis:

echo KVM_PYTHON=python3.9 >> Makefile.inc.local

Create the Pool directory for storing VM disk images - $(KVM_POOLDIR)

The pool directory is used used to store:

  • VM disk images
  • install CD/DVD images
  • downloaded packages installed into the VMs
  • other files

and can get quite large. It can and should be shared between build trees (this reflects libvirt which has a single name space for domains). KVM_PREFIXES (see further down) addresses the lack of name spaces.

By default $(top_srcdir)/../pool (../pool) is used (that is, adjacent to your source tree). It will need to be created.

Alternatively the shared pool directory can be specified explicitly by setting the make variable KVM_POOLDIR in Makefile.inc.local vis:

mkdir KVM_POOLDIR=/home/libreswan/pool
echo KVM_POOLDIR=/home/libreswan/pool >> Makefile.inc.local

(optional) Use /tmp/pool (tmpfs) to store test VM disk images - $(KVM_LOCALDIR)

By default, all disk mages are stored in $(KVM_POOLDIR) (see above). Since the test VM disk images do not need long-term storage (i.e., survive a reboot), $(KVM_LOCALDIR) can be used to specify that test VM disk images are stored in /tmp vis:

echo KVM_LOCALDIR=/tmp/pool >> Makefile.inc.local

This has the advantage of eliminating physical disk I/O as a bottle neck when accessing VM disk images; but the disadvantage of needing to re-build the test disk images after a reboot.

(optional) Define test groups and run tests in parallel - $(KVM_PREFIXES)

By default there is only one set of build and test domains. This means that a host can only support one build tree, and that tests are run sequentially.

This can be changed by setting the KVM_PREFIXES make variable in Makefile.inc.local. This specifies a list of prefixes to be pretended to build and test domains creating multiple test groups.

The default value is:


which creates the build domains fedora-base, fedora-upgrade, fedora, et.al., and the test domains east, west, et.al. (i.e., after expansion fedora-base, fedora-upgrade, fedora, east, west, et.al.).

To run tests in parallel and assign unique domains to the build directory, specify multiple prefixes. For instance a build tree can be given unique domains and run two tests in parallel by specifying:


This will create the build domains t1fedora-base, t1fedora-upgrade, t1fedora, et.al., and the test domains t1east, t2east, et.al.

Due to limitations in the network stack (networks with prefix t. and t2. are also created) the prefix needs to be short.

(optional) Parallel builds - $(KVM_WORKERS)

By default, build domains only have on virtual CPU. Since building is very CPU intensive, this can be increased using $(KVM_WORKERS).


(optional) Generate a web page of the test results

See the nightly test results for an example and how to set up a web server so results can be viewed remotely.

To initially create the web directory RESULTS/ and populate it with the current test results use:

make web

Further test runs will update the RESULTS/ directory. The files can the be viewed using http://file.

To disable web page generation, delete the directory RESULTS/.

To instead publish the results on the web, point $(WEB_SUMMARYDIR) at the web directory:

$ WEB_SUMMARYDIR=/var/www/html/results >> Makefile.inc.local

Running the testsuite

In the past, the testsuite was driven using make kvm-... commands. That's largely been replaced by the top-level wrapper script ./kvm which has several advantages over make:

  • it is file (file) completion friendly
  • it is shell script friendly

For the impatient: ./kvm install check

To build the VMs, and build and install (or update) libreswan, and then run the tests, use:

./kvm install check

Setting up ./kvm (tab completion)

If this:

complete -o filenames -C './kvm' ./kvm

is added to .bashrc then tab completion with ./kvm will include both commands and directories.

Running the testsuite

./kvm install
update the KVMs ready for a new test run
./kvm check
run the testsuite, previous results are saved in BACKUP/-date-
./kvm recheck
run the testsuite, but skip tests that already passed
./kvm results
list the results from the test run
./kvm diffs
display differences between the test results and the expected results, exit non-zero if there are any

the operations can be combined on a single line:

./kvm install check recheck diff

and individual tests can be selected (see Running a Single Test, below):

./kvm install check diff testing/pluto/*ikev2*

To stop ./kvm use control-c.

Updating Certificates

The full testsuite requires a number of certificates. If not present, then ./kvm check will automatically generate them using the domain build. Just note that the certificates have a limited lifetime. Should the test system detects out-of-date certificates then ./kvm check will barf.

To rebuild the certificates:

./kvm keys

can be used to force the generation of new certificates.

Cleaning up (and general maintenance)

./kvm check-clean
delete the test results
./kvm uninstall
delete the KVM build and test domains (but don't touch the build tree or test results)
./kvm clean
delete the test results, the KVM build and test domains, the build tree, and the certificates
./kvm purge
also delete the test networks (is purge still useful?)
./kvm demolish
also delete the KVM base domain that was used to create the other domains
./kvm upgrade
delete all KVM build and test domains, and then upgrade and transmogrify the base domain ready for a fresh install
./kvm transmogrify
run a fresh transmogrify on the base domain (the base domain is reverted to before the last transmogrify)
./kvm downgrade
revert the base domain back to before it was upgraded (useful when debugging upgrade and transmogrify)

Shell and Console Access (Logging In)

There are several different ways to gain shell access to the domains.

Each method, depending on the situation, has both advantages and disadvantages. For instance:

  • while make kvmsh-host provide quick access to the console, it doesn't support file copy
  • while SSH takes more to set up, it supports things like proper terminal configuration and file copy

Serial Console access using ./kvm sh HOST (kvmsh.py)

./kvm sh HOST is a wrapper around "virsh" that automatically handles things like booting the machine, logging in, and correctly configuring the terminal. It's big advantage is that it always works. For instance:

$ ./testing/utils/kvmsh.py east
Escape character is ^]
[root@east ~]# printenv TERM
[root@east ~]# stty -a
...; rows 52; columns 185; ... 
[root@east ~]#

The script "kvmsh.py" can also be used directly to invoke commands on a guest (this is how ./kvm install works):

$ ./testing/utils/kvmsh.py east ls
[root@east ~]# ls

When $(KVM_PREFIXES) contains multiple prefixes, ./kvm sh east always logs into the first prefixe's domain.


  • no file transfer but files can be accessed via /pool and /testing

Graphical Console access using virt-manager

"virt-manager", a gnome tool can be used to access individual domains.

While easy to use, it doesn't support cut/paste or mechanisms for copying files.

Shell access using SSH

While requiring more effort to set up, it provides full shell access to the domains.

Since you will be using ssh a lot to login to these machines, it is recommended to either put their names in /etc/hosts:

# /etc/hosts entries for libreswan test suite west east north road nic

or add entries to .ssh/config such as:

Host west

If you wish to be able to ssh into all the VMs created without using a password, add your ssh public key to testing/baseconfigs/all/etc/ssh/authorized_keys. This file is installed as /root/.ssh/authorized_keys on all VMs

Using ssh becomes easier if you are running ssh-agent (you probably are) and your public key is known to the virtual machine. This command, run on the host, installs your public key on the root account of the guest machines west. This assumes that west is up (it might not be, but you can put this off until you actually need ssh, at which time the machine would need to be up anyway). Remember that the root password on each guest machine is "swan".

ssh-copy-id root@west

You can use ssh-copy for any VM. Unfortunately, the key is forgotten when the VM is restarted.


  • this only works with the default east, et.al. (it does not work with KVM_PREFIXES and/or multiple test directories)

kvm workflows

(seeing as everyone has a "flow", why not kvm) here are some common workflows, the following commands are used:

./kvm modified
list the test directories that have been modified
./kvm baseline
compare test results against a baseline
./kvm patch
update the expected test results
./kvm add
git add the modified test results
./kvm status
show the status of the currently running testsuite
./kvm kill
kill the currently running testsuite

Running a single test

There are two ways to run an individual test:

  1. the test to run can be specified on the command line:
    kvm check testing/pluto/basic-pluto-01
  2. the test is implied when running kvm from a test directory:
    cd testing/pluto/basic-pluto-01
    ../../../kvm diff

But there's a catch:

  • in batch mode pluto is shutdown at the end of the test
this way additional post-mortem checks, such as for memory leaks and core dumps that rely on pluto being stopped, can be performed
  • in single test mode the system is left running
this way it is possible to log in and look around the running system and attach a debugger to pluto before it is shutdown

To instead force post-mortem, add:

KVMRUNNER_FLAGS += --run-post-mortem

to Makefile.inc.local.

Working on individual tests

The modified command can be used to limit the test run to just tests with modified files (according to git):

./kvm modified install check diff
install libreswan and then run the testsuite against just the modified tests, display differences differences
./kvm modified recheck diff
re-run the modified tests that are failing, display differences
./kvm modified patch add
update the modified tests applying the latest output and add them to git

this workflow comes into its own, when updating tests en-mass using sed, for instance:

sed -i -e 's/PARENT_//' testing/pluto/*/*.console.txt
./kvm modified check

Comparing results to a baseline

Note: since all "good" tests should pass, this workflow is largely redundant.

Start by setting up a baseline directory. Give the KVMs unique bN prefixes (only "b1" is needed, but we're in a hurry so add "b2 b3 b4", 4 boot workers, and /tmp/pool for KVM disk images) and kick off a test run:

$ git clone https://github.com/libreswan/libreswan baseline
$ cd baseline
baseline$ # baseline - use bN as the prefix
baseline$ echo KVM_PREFIXES=b1 b2 b3 b4 >> base/Makefile.inc.local
baseline$ echo KVM_WORKERS=4            >> base/Makefile.inc.local
baseline$ echo KVM_LOCALDIR=/tmp/pool   >> base/Makefile.inc.local
baseline$ mkdir -p ../pool
baseline$ nohup ./kvm install check &
baseline$ tail -f nohup.out

Next, set up a working directory. This time the KVMs are given the unique wN prefix, and point KVM_BASELINE back at the baseline/ directory:

$ git clone https://github.com/libreswan/libreswan work
$ cd work
work$ # work - use wN as the prefix
work$ echo KVM_PREFIXES=w1 w2 w3 w4 >> work/Makefile.inc.local
work$ echo KVM_WORKERS=4            >> work/Makefile.inc.local
work$ echo KVM_BASELINE=../baseline >> work/Makefile.inc.local
work$ echo KVM_LOCALDIR=/tmp/pool   >> work/Makefile.inc.local
work$ mkdir -p ../pool

work then then progress in the work directory, and when ready the test run started (here in the background):

work$ ed programs/pluto/plutomain.c
/static bool selftest_only = false/ s/false/true/
work$ gmake && nohup ./kvm install check &

as the tests progress, the results can be monitored:

work$ ./kvm baseline results
testing/pluto/basic-pluto-01 failed east:baseline-passed,output-different west:baseline-passed,output-different
work$ ./kvm baseline diffs testing/pluto/basic-pluto-01
+whack: Pluto is not running (no "/run/pluto/pluto.ctl")

and then the test run aborted, and the problem fixed and tested, and the test run restarted:

work$ ./kvm kill
work$ git checkout -- programs/pluto/plutomain.c
work$ ./kvm install check diff testing/pluto/basic-pluto-01
work$ nohup recheck &

The output can be fine tuned using baseline-failed (show differences when the baseline failed, ignoring passed and unresolved) baseline-passed (show differences when the baseline passed, ignoring failed and unresolved).

To override the KVM_BASELINE make variable, use --baseline DIRECTORY

Controlling a test run remotely

Start the testsuite in the background:

nohup ./kvm install check &

To determine if the testsuite is still running:

./kvm status

and to stop the running testsuite:

./kvm kill

Debugging inside the VM (pluto on east)

Terminal 1 - east: log into east, start pluto, and attach gdb

./kvm sh east
east# cd /testing/pluto/basic-pluto-01
east# sh -x ./eastinit.sh
east# gdb /usr/local/libexec/ipsec/pluto $(pidof pluto)
(gdb) c

If pluto isn't running then gdb will complain with: --p requires an argument

Terminal 2 - west: log into west, start pluto and the test

./kvm sh west
west# sh -x ./westinit.sh ; sh -x westrun.sh

When pluto crashes, gdb will show that and await commands. For example, the bt command will show a backtrace.


  • stop watchdog eventually killing pluto
  • notes for west

Installing a custom Fedora kernel

Assuming the kernel RPMs are in the directory $(KVM_POOLDIR)/kernel-ipsec/ say, add the following to Makefile.inc.local:

KVM_FEDORA_KERNEL_RPMDIR = /pool/kernel-ipsec/
KVM_FEDORA_KERNEL_VERSION = -5.18.7-100.aiven_ipsec.fc35.$(KVM_FEDORA_KERNEL_ARCH).rpm

and then run:

./kvm upgrade-fedora

(should, like for NetBSD do this during transmogrify?)

Installing a custom NetBSD kernel

Copy the kernel to:


and then run:

./kvm transmogrify-netbsd

Building and testing an old branch

Old branches have two problems:

  • the KVM codebase is out-of-date
  • the OS releases are gone

Here are two ways to get around it:

Using a test-bench

This workflow works best when working on an old branch (lets say v4.11)

Two repositories are used:

  1. repo under test aka RUTDIR
    this contains both the sources and the tests
  2. testbench
    this contains the test scripts used to drive ${RUT}

Start by checking out the two repositories (existing repositories can also be used, carefully):

RUTDIR=$PWD/v4_maint ; export RUTDIR
git clone https://github.com/libreswan/libreswan.git -r v4_maint ${RUTDIR}
git clone https://github.com/libreswan/libreswan.git testbench

Next, configure testbench so that it compiles, installs, and runs tests from ${RUTDIR} by setting the $(KVM_RUTDIR) make variable:

echo KVM_RUTDIR=$(realpath $RUTDIR)           >> testbench/Makefile.inc.local

($(KVM_SOURCEDIR) and $(KVM_TESTINGDIR) default to $(KVM_RUTDIR); you can also set $(KVM_SOURCEDIR) and $(KVM_TESTINGDIR) explicitly).

Now, (re-)transmogrify the testbench so that, within the domains, /source points at ${RUT} and /testing points at ${RUT}/testing:

./testbench/kvm transmogrify

in the command building the fedora domain look for output like:

--filesystem=target=bench,type=mount,accessmode=squash,source=/.../testbench \
--filesystem=target=source,type=mount,accessmode=squash,source=${RUTDIR} \
--filesystem=target=testing,type=mount,accessmode=squash,source=${RUTDIR}/testing \

Finally install and then run a test:

./testbench/kvm install check diff $RUT/testing/pluto/basic-pluto-01

If you prefer you can run testbench/kvm:

  • from the testbench directory as ./kvm
  • from the ${RUTDIR} directory as ../testbench/kvm

just do not run $RUTDIR/kvm.

Reviving the dead OS

Again looking at v4_maint branch. Check it out:

 git checkout ... -b v4_maint

add the following to Makefile.inc.local:

 KVM_FEDORA_ISO_URL = https://archives.fedoraproject.org/pub/archive/fedora/linux/releases/35/Server/x86_64/iso/Fedora-Server-dvd-x86_64-35-1.2.iso

build fedora-base:

 ./kvm base-fedora

login to the base domain:

 ./kvm sh fedora-base

and edit the repos per:

 /etc/yum.repos.d/fedora.repo:name=Fedora $releasever - $basearch
 /etc/yum.repos.d/fedora.repo:name=Fedora $releasever - $basearch - Debug
 /etc/yum.repos.d/fedora.repo:name=Fedora $releasever - Source
 /etc/yum.repos.d/fedora-updates.repo:name=Fedora $releasever - $basearch - Updates
 /etc/yum.repos.d/fedora-updates.repo:name=Fedora $releasever - $basearch - Updates - Debug
 /etc/yum.repos.d/fedora-updates.repo:name=Fedora $releasever - Updates Source

after that:

 ./kvm install check

might work

Tracking down regressions (using git bisect)

The easy way

This workflow works best when the regression is recent (i.e., the last few commits) and nothing significant has happened in the meantime (for instance, os upgrade, test rename, ...).

The command ./kvm install check diff exits with a git bisect friendly status codes which means it can be combined with git bisect run to automate regression testing.

For instance:

git bisect start main ^<suspect-commit>
git bisect run ./kvm install check diff testing/pluto/basic-pluto-01
git bisect visualize
# finally
git bisect reset

The hard way

This workflow works best when trying to track down a regression in an older version of libreswan.

Two repositories are used:

  1. repo-under-test
    this contains the sources that will be built and installed into the test domains and is what git bisect will manipulate
  2. testbench
    this contains the test scripts used to drive repo-under-test

Start by checking out the two repositories (existing repositories can also be used, carefully):

git clone https://github.com/libreswan/libreswan.git repo-under-test
git clone https://github.com/libreswan/libreswan.git testbench

and then cd to the repo-under-test directory:

 cd repo-under-test

Next, configure testbench so that it compiles and installs libreswan from repo-under-test but runs tests from testbench. Do this by pointing the testbench KVM_SOURCEDIR (/source) at repo-under-test vis:

# remember $PWD is repo-under-test
echo KVM_SOURCEDIR=$(realpath ../repo-under-test)    >>../testbench/Makefile.inc.local
echo KVM_TESTINGDIR=$(realpath ../testbench/testing) >>../testbench/Makefile.inc.local

Now, (re-)transmogrify the testbench so that, within the domains, /source points at repo-under-test:

../testbench/kvm transmogrify

in the command building the fedora domain look for output like:

--filesystem=target=bench,type=mount,accessmode=squash,source=/.../testbench \
--filesystem=target=source,type=mount,accessmode=squash,source=/.../repo-under-test \
--filesystem=target=testing,type=mount,accessmode=squash,source=/.../testbench/testing \

Finally run the tests (remember testing/pluto/basic-pluto-01 is the test that started failing):

# start with the bad commit
git bisect start main
# next checkout and confirm the good commit
# NOTE: run testbench/kvm from repo-under-test directory
git checkout <good-commit>
../testbench/kvm install check diff testing/pluto/basic-pluto-01
git bisect good

if you're lucky, the test requires no manual intervention and:

git bisect run ../testbench/kvm install check diff testing/pluto/basic-pluto-01

also works:

# finally
git bisect visualize
git bisect reset

TODO: figure out how to get ../testbench/kvm diff to honour KVM_TESTINGDIR so that it can handle a test somewhere other than in <tt?testbench