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Last pushed: 3 days ago
Short Description
Solo5 MBR-partitioned disk image builder for virtio unikernels
Full Description
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About Solo5

Solo5 is most useful as a "base layer" to run
MirageOS unikernels, either on various existing
hypervisors (KVM/QEMU, bhyve) or on a specialized "unikernel monitor" called

About ukvm

ukvm runs as a Linux process and uses KVM. The goal of ukvm is to
be a small, modular monitor, in which its functionality and interfaces
are customized to the unikernel that is being built. In other words,
the unikernel monitor exhibits the characteristic of "only what is
needed" for the unikernel to run. It has the potential to provide a
thinner interface to the guest unikernel (thinner than either a
container or a VM), a simpler I/O interface (e.g., packet send rather
than virtio), and better performance due to its simplicity (e.g., fast
boot). Check out our
from USENIX HotCloud '16 for more information.

Building MirageOS unikernels with Solo5

Support for Solo5 as a target is integrated since the release of MirageOS 3.0,
which adds two new targets to the mirage configure command:

  1. ukvm: A specialized "unikernel monitor" which runs on Linux
    (x86_64) and uses KVM directly via /dev/kvm.
  2. virtio: An x86_64 system with virtio network and disk
    interfaces. Use this target for QEMU/KVM, plain QEMU, bhyve or other
    hypervisors (see below).

You can build for either of these targets using the standard MirageOS build
process, which will output either a .ukvm or .virtio unikernel binary.

For the ukvm target, in addition to the .ukvm unikernel binary a ukvm-bin
binary will be built. This is the ukvm monitor specialised for your

Running Solo5 unikernels directly

The following examples use the standalone
test_ping_serve unikernel which is
built as part of the normal Solo5 build process.

test_ping_serve is a minimalist network test which will respond only to ARP
and ICMP echo requests sent to the hard-coded IP address of It
accepts two possible command line arguments: Either verbose for verbose
operation or limit to terminate after having sent 100,000 ICMP echo replies.

Setting up

By convention, we will use the tap100 interface to talk to the unikernel.

To set up the tap100 interface on Linux, run (as root):

ip tuntap add tap100 mode tap
ip addr add dev tap100
ip link set dev tap100 up

To set up bhyve and the tap100 interface on FreeBSD, run (as root):

kldload vmm
kldload if_tap
kldload nmdm
sysctl -w
ifconfig tap100 create link0 up

Running with ukvm on Linux

A specialized monitor called ukvm-bin is generated as part of the
build process, so the command line arguments may differ depending on
the needs of the unikernel. To see the arguments, run ukvm-bin with
no arguments or --help.

To launch the unikernel:

./ukvm-bin --net=tap100 ./test_ping_serve.ukvm verbose

Use ^C to terminate the unikernel.

Running with KVM/QEMU on Linux, or bhyve on FreeBSD

To launch the unikernel:

solo5-run-virtio -n tap100 ./test_ping_serve.virtio verbose

Use ^C to terminate the unikernel.

The solo5-run-virtio script is automatically
installed in your $PATH when using the solo5-kernel-virtio OPAM package as
part of a MirageOS build environment.
Using it is not required; by default it will print the commands used to setup
and launch the guest VM. You can run these manually if desired.

Running virtio unikernels with other hypervisors

The virtio target produces a unikernel that uses the multiboot
protocol for booting. If your hypervisor can boot a multiboot-compliant
kernel directly then this is the preferred method.

If your hypervisor requires a full disk image to boot, you can use the
solo5-mkimage tool to build one. This tool is
automatically installed in your $PATH when using the solo5-kernel-virtio
OPAM package as part of a MirageOS build environment.

solo5-mkimage supports the following image formats:

  • raw: A raw disk image, written out as a sparse file.
  • tar: A disk image suitable for uploading to Google Compute Engine.

The following devices are supported by the Solo5 virtio target:

  • the serial console, fixed at COM1 and 115200 baud
  • the KVM paravirtualized clock, if available
  • a single virtio network device attached to the PCI bus
  • a single virtio block device attached to the PCI bus

Note that Solo5 on virtio does not support ACPI power-off. This can manifest
itself in delays shutting down Solo5 guests running on hypervisors which wait
for the guest to respond to ACPI power-off before performing a hard shutdown.

Developing Solo5 and ukvm

If you'd like to develop Solo5, ukvm and/or investigate porting other
unikernels to use Solo5 as a base layer, the public APIs are defined in
kernel/solo5.h and ukvm/ukvm.h. These interfaces are still evolving
and subject to change.

We also have some simple standalone unikernels written in C to test
Solo5, see tests for these.

The coding style for this project is "as for the Linux kernel, but with 4
spaces instead of tabs".

The best place for Mirage-related discussions about Solo5 and/or
ukvm is to post to the MirageOS-devel mailing list,
with general Solo5/ukvm discussions on

Debugging on ukvm

You can debug the unikernel running in ukvm using gdb, but need to
build a ukvm-bin with gdb support. To do so, edit the Makefile
(generated in the case of Mirage), and add gdb to the UKVM_MODULES
make variable. Then, running make ukvm-bin should build a version
with gdb support.

Start ukvm with the --gdb flag, like this:

./ukvm-bin mir-console.solo5-ukvm --gdb

And then from another console start gdb and connect to the remote target
listening at localhost:1234:

$ gdb mir-console.solo5-ukvm

(gdb) target remote localhost:1234

Here is a typical gdb session:

(gdb) target remote localhost:1234
Remote debugging using localhost:1234
kernel_main (size=536870912, kernel_end=2625536) at kernel.c:36
36    void kernel_main(uint64_t size, uint64_t kernel_end) {
(gdb) break
Breakpoint 11 at 0x104c39: file, line 9.
(gdb) c

Breakpoint 11, camlUnikernel__loop_1401 () at
9            C.log c "hello";
(gdb) bt
#0  camlUnikernel__loop_1401 () at
#1  0x000000000010480e in camlMain__fun_1487 () at
#2  0x0000000000178ad9 in camlCamlinternalLazy__force_lazy_block_1010 () at
#3  0x00000000001049e4 in camlMain__fun_1494 () at
#4  0x0000000000178ad9 in camlCamlinternalLazy__force_lazy_block_1010 () at
#5  0x0000000000104bfa in camlMain__entry () at src/core/
#6  0x0000000000100ed9 in caml_program ()
#7  0x00000000001c26a6 in caml_start_program ()
#8  0x0000000000000000 in ?? ()
(gdb) s

Breakpoint 11, camlUnikernel__loop_1401 () at
9            C.log c "hello";
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