Wednesday, June 10, 2015

Rapid Linux Kernel Dev/Test with QEMU, KVM and Dracut

Update 2018-01-02: See my post on Rapido for a more automated approach to the procedure outlined below.

Inspired by Stefan Hajnoczi's excellent blog post, I recently set about constructing an environment for rapid testing of Linux kernel changes, particularly focused on the LIO iSCSI target. Such an environment would help me in number of ways:
  • Faster dev / test turnaround.
    • A modified kernel can be compiled and booted in a matter of seconds.
  • Improved resource utilisation.
    • No need to boot external test hosts or heavyweight VMs.
  •  Simplified and speedier debugging.

My requirements were slightly different to Stefan's, in that:
  • I'd prefer to be lazy and use Dracut for initramfs generation.
  • I need a working network connection between VM and hypervisor system
    • The VM will act as the iSCSI target, the hypervisor as the initiator.

Starting with the Linux kernel, the first step is to build a bzimage:
~/> git clone \
hack, hack, hack.
~/linux/> make menuconfig
Set CONFIG_IP_PNP_DHCP=y and CONFIG_E1000=y to enable IP address assignment on boot.
~/linux/> make -j6
~/linux/> make modules
~/linux/> INSTALL_MOD_PATH=./mods make modules_install
~/linux/> sudo ln -s $PWD/mods/lib/modules/$(make kernelrelease) \
                     /lib/modules/$(make kernelrelease)
This leaves us with a compressed kernel image file at arch/x86/boot/bzimage, and corresponding modules installed under mods/lib/module/$(make kernelrelease), where $(make kernelrelease) evaluates to 4.1.0-rc7+ in this example. The /lib/modules/4.1.0-rc7+ symlink allows Dracut to locate the modules.

The next step is to generate an initial RAM filesystem, or initramfs, which includes a minimal set of user-space utilities, and kernel modules needed for testing:

~/linux/> dracut --kver "$(make kernelrelease)" \
                 --add-drivers "iscsi_target_mod target_core_mod" \
                 --add-drivers "target_core_file target_core_iblock" \
                 --add-drivers "configfs" \
                 --install "ps grep netstat" \
                 --no-hostonly --no-hostonly-cmdline \
                 --modules "bash base shutdown network ifcfg" initramfs
*** Creating image file done ***

We now have an initramfs file in the current directory, with the following contents:
  • LIO kernel modules obtained from /lib/module/4.1.0-rc7, as directed via the --kver and --add-drivers parameters.
  • User-space shell, boot and network helpers, as directed via the --modules parameter.

We're now ready to use QEMU/KVM to boot our test kernel and initramfs:

~/linux/> qemu-kvm -kernel arch/x86/boot/bzImage \
                   -initrd initramfs \
                   -device e1000,netdev=network0 \
                   -netdev user,id=network0 \
                   -redir tcp:51550::3260 \
                   -append "ip=dhcp console=ttyS0" \

This boots the test environment, with the kernel and initramfs previously generated:

[    3.216596] dracut Warning: dracut: FATAL: No or empty root= argument
[    3.217998] dracut Warning: dracut: Refusing to continue
Dropping to debug shell.


From the dracut shell, confirm that the QEMU DHCP server assigned the VM an IP address:

dracut:/# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default 
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    inet brd scope global eth0

Port 3260 (iSCSI) on this interface is forwarded to/from port 51550 on the hypervisor, as configured via the qemu-kvm -redir parameter.

Now onto LIO iSCSI target setup. First off load the appropriate kernel modules:

dracut:/# modprobe iscsi_target_mod
dracut:/# cat /proc/modules
iscsi_target_mod 246669 0 - Live 0xffffffffa006a000
target_core_mod 289004 1 iscsi_target_mod, Live 0xffffffffa000b000
configfs 22407 3 iscsi_target_mod,target_core_mod, Live 0xffffffffa0000000

LIO configuration requires a mounted configfs filesystem:

dracut:/# mount -t configfs configfs /sys/kernel/config/
dracut:/# cat /sys/kernel/config/target/version 
Target Engine Core ConfigFS Infrastructure v4.1.0 on Linux/x86_64 on 4.1.0-rc1+

An iSCSI target can be provisioned by manipulating corresponding configfs entries. I used the lio_dump output on an existing setup as reference:

dracut:/# mkdir /sys/kernel/config/target/iscsi
dracut:/# echo -n 0 > /sys/kernel/config/target/iscsi/discovery_auth/enforce_discovery_auth
dracut:/# mkdir -p /sys/kernel/config/target/iscsi/<iscsi_iqn>/tpgt_1/np/

Finally, we're ready to connect to the LIO target using the local hypervisor port that forwards to the VM's virtual network adapter:

~/linux/> iscsiadm --mode discovery \
                   --type sendtargets \
                   --portal,1 iqn.2015-04.suse.arch:5eca2313-028d-435c-9131-53a5ab256a83

It works!

There are a few things that can be adjusted:
  • Port forwarding to the VM network is a bit fiddly - I'm now using a bridge/TAP configuration instead.
  • When dropping into the emergency boot shell, Dracut executes scripts carried under /lib/dracut/hooks/emergency/. This means that a custom script can be triggered on boot via:
    ~/linux/> dracut -i /lib/dracut/hooks/emergency/ ...
  • It should be possible to have Dracut pull the kernel modules in from the temporary directory, but I wasn't able to get this working:
    ~/linux/> INSTALL_MOD_PATH=./mods make modules_install
    ~/linux/> dracut --kver "$(make kernelrelease)" --kmoddir ./mods/lib/...
  • Boot time and initramfs file IO performance can be improved by disabling compression. This is done by specifying the --no-compress Dracut parameter.

Update 20150722:
  • Don't install kernel modules as root, set up a /lib/modules symlink for Dracut instead.
  • Link to bridge/TAP networking post.
  • Describe boot script usage.
Update 20150813:
  • Use $(make kernelrelease) rather than a hard-coded 4.1.0-rc7+ kernel version string - thanks Aurélien!
Update 20150908: Describe initramfs --no-compress optimisation.
Update 20180102: Link to the Rapido project.


  1. I'm also having success with this Buildroot setup:


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