XPDDS19: A Journey through Unikraft's Build System

Simon Kuenzer <simon.kuenzer@neclab.eu>
Senior Researcher, NEC Laboratories Europe GmbH
Xen Summit 2019, Chicago
A Journey through Unikraft’s Build System
This work has received funding from the European Union’s Horizon 2020 research and innovation
program under grant agreements no. 675806 (“5G CITY”). This work reﬂects only the author’s views
and the European Commission is not responsible for any use that may be made of the information it
contains.

Unikraft‘s Build System
Overview

4 © NEC Corporation 2019
Unikraft
KConfig
Makefile System
Internal Library
Config.uk
Makefile.uk
Sources
Build System: Loading & Parsing
Your Application
Config.uk
Makefile.uk
Sources
Makefile
External Library
Config.uk
Makefile.uk
Sources

Unikraft’s 3 Build Stages
(1) Fetch
Download and decompress external sources
e.g., a library hosted on GitHub, Sourceforge
Patch downloaded files
(2) Prepare
Further preparation steps to the sources, for instance:
• Call ./configure of downloaded library sources
• Generate further sources or headers required for building
(3) Compile & Link
Compile sources
Link libraries
Link final images

*_SRCS-y *_OBJS-y UK_ALIBS-y
UK_OLIBS-y
UK_IMAGES-y
Build Stage 3: Compiling and Linking
libA_a.c
libA_b.S libA.ld.o libA.o
... libB.o
Images
prebuilt.o
...
staticA.a
...
exportsyms.uk
libA_a.o
libA_b.o
......
extA.o
...

Make parameters
▌Unikraft’s base Makefile
$ make A=[APP] L=[LIBRARIES] P=[PLATFORMS] V=[1/0] [target]
Parameter Description
A=[APP] Path to application directory
L=[LIBRARIES] Colon-separated list of paths to external libraries
P=[PLATFORMS] Colon-separated list of paths to external platform libs
V=[1/0] Verbose mode (on/off)
[target] Build target
help Show overview of targets
menuconfig Configure and select target images
(default when there is no .config)
all/images Build everything (default) +libs
libs Build libraries +prepare
prepare Run preparations steps +fetch
fetch Download, extract, and patch external code

Unikraft Libraries
Integrate own libraries and applications

Libraries/Applications/Platforms: Necessary files
▌Makefile *applications only
 Invoke Unikraft build for simplification
▌Config.uk
 Configuration options
• Settings saved as part of .config
 Specifying library dependencies
and depending options
▌Makefile.uk
 Registration to the build system
 Specification of source files
 Extra custom Make rules
• For instance for preparing the sources
▌exportsyms.uk
 Masking of symbols
▌Linker.uk *platform libraries only
 Platform-dependent rules for linking final image
.config

Makefile *applications only
▌Simplify application building
▌Changes to Unikraft base directory and invokes make
UK_ROOT ?= $(PWD)/../../unikraft
UK_LIBS ?= $(PWD)/../../libs
LIBS := $(UK_LIBS)/libA:$(UK_LIBS)/libB
all:
@$(MAKE) -C $(UK_ROOT) A=$(PWD) L=$(LIBS)
$(MAKECMDGOALS):
@$(MAKE) -C $(UK_ROOT) A=$(PWD) L=$(LIBS) (MAKECMDGOALS)

Config.uk
▌KConfig syntax1
▌Structure:
Title and Dependencies
Configuration parameters
(optional)
[1] https://www.kernel.org/doc/Documentation/kbuild/kconfig-language.txt

Config.uk: Title and Dependencies for Libraries
menuconfig defines a submenu for the following configuration options
• Note: Use just config without following if-block when the library does not
have any configuration parameters
select keyword is used to describe dependencies to other libraries
• Supports conditional expressions1
• Multiple select lines are possible for a single configuration
menuconfig LIBMYLIB
bool "ukmylib: my scheduler“
### libraries are off as default
default n
### dependencies
select LIBNOLIBC if !HAVE_LIBC
if LIBMYLIB
### list of configuration parameters goes here
endif

Config.uk: Dependencies for Applications
Invisible bool config to select dependencies
• Applications are not defining an own submenu. Because just a single
application can currently be selected for one Unikernel build, applications are
enabled as default
Like for libraries, select keyword is used to describe dependencies to
libraries
### Invisible option (no description) that is set as
### default to ‚y‘ in order to select dependencies
config LIBMYAPP
bool
default y
select LIBNOLIBC if !HAVE_LIBC
### list of configuration parameters goes here

Config.uk: Configuration Parameter
Name-space configuration options!
• Prepend library name in front of parameters: here: LIBMYLIB_
Configurations will appear as CONFIG_[CONFIGNAME]
in the build system and in the sources (include uk/config.h)
(here: CONFIG_LIBMYLIB_SETTING)
[type] can be one of bool, int (unsigned), hex, string:
Advanced options, like choice lists, are documented at:
https://www.kernel.org/doc/Documentation/kbuild/kconfig-language.txt
config LIBMYLIB_SETTING
[type] "[description]"
default [value]
select LIBOTHER
Makefile.uk #include <uk/config.h>
bool y/n „y“ is defined as 1
„n“ is not defined
int Hexadecimal Defined as hexadecimal
hex Hexadecimal Defined as hexadecimal
string String Defined as String

Makefile.uk
▌Makefile syntax
Unikraft provides helper functions and variables
Unikraft expects specific variables to be filled
▌Structure
Registration
Source files and build flags
Custom prepare rules
External sources (optional)

Makefile.uk: Registration
▌Registration with addlib / addlib_s helpers1
The first thing that has to be done in a Makefile.uk
Libraries (depending on being enabled)
Replace [libmylib] and CONFIG_LIBMYLIB accordingly:
Applications
Replace [libmyapp] accordingly:
The namespace for variables is defined by application/library name
here: prefixed in uppercase: LIBMYLIB_, LIBMYAPP_
The following variables are populated after the call
The following overrides are available after the call
$(eval $(call addlib_s,[libmylib],$(CONFIG_LIBMYLIB)))
[1] support/build/Makefile.rules
*_BASE Path to library folder
*_BUILD Path to library‘s output/build folder
Note: Place all generated files during building in here, never in the base
$(eval $(call addlib,[libmyapp]))
*_EXPORTS Path to an alternative exportsyms.uk (optional)

Makefile.uk: External sources (optional)
▌Download one archive with (additional) sources and extract them
with fetch / fetchas helpers1
Example with lwIP:
.tar.gz, .tgz, .tar.xz, .txz, and .zip are currently supported1
The following variables are populated after this call
▌If some downloaded source files need to be patched, use
patch helper1:
Example with lwIP:
This command applies all patches found in $(LIBLWIP_PATCHDIR) to the sub-
directory $(LIBLWIP_ZIPNAME) of the previously extracted sources
LIBLWIP_ZIPNAME=lwip-2.1.2
LIBLWIP_URL=http://download.savannah.nongnu.org/releases/lwip/$(LIBLWIP_ZIPNAME).zip
$(eval $(call fetch,liblwip,$(LIBLWIP_URL)))
*_ORIGIN Path to folder containing extracted archive files
LIBLWIP_PATCHDIR=$(LIBLWIP_BASE)/patches
$(eval $(call patch,liblwip,$(LIBLWIP_PATCHDIR),$(LIBLWIP_ZIPNAME)))

Makefile.uk: Source files
▌Full paths to source files are added to the *_SRCS-y list:
Compile rule and target is automatically generated by build system
based on file extension:
• Currently supported: .S, .sx, .s, .c, .C, .cc, .cp, .cxx, .cpp, .CPP, .c++, .lds.S
• .o-binary is created within the library output directory based on the source filename:
– Note: Only the filename without extension matters for the target file name. The source file extension
and path is irrelevant. In conflicting cases, use variants (see next slide).
The following include and build flag lists apply for a source file:
# Source file from library directory
LIBMYLIB_SRCS-y += $(LIBMYLIB_BASE)/source.c
# Source file from extracted archive
LIBMYLIB_SRCS-y += $(LIBMYLIB_ORIGIN)/another_src.c
CFLAGS-y Global build flags (here: C sources, equivalents for other types exist)
CINCLUDES-y Global includes (here: C sources, equivalents for other types exist)
*_CFLAGS-y Library-internal build flags (here: C sources, equivalents exist)
*_CINCLUDES-y Library-internal includes (here: C sources, equivalents exist)
*_[FILENAME]_FLAGS Source file specific build flags
*_[FILENAME]_INCLUDES Source file specific includes
$(LIBMYLIB_BASE)/source.c.  $(LIBMYLIB_BUILD)/source.o
$(LIBMYLIB_ORIGIN)/another_src.c  $(LIBMYLIB_BUILD)/another_src.o

Makefile.uk: Source File Variants
▌Variants exist because of two reasons:
Conflicting output file names (previous slide)
Necessity to compile a single source file multiple times with different flags
(e.g., newlib *scanf() variants)
▌Variant names are added with a pipe symbol after source:
…produces:
Variants have their own specific build flags and includes:
LIBMYLIB_SRCS-y += $(LIBMYLIB_BASE)/source.c
LIBMYLIB_SRCS-y += $(LIBMYLIB_BASE)/source.c|variant
LIBMYLIB_SRCS-y += $(LIBMYLIB_ORIGIN)/source.c|origin
$(LIBMYLIB_BASE)/source.c  $(LIBMYLIB_BUILD)/source.o
$(LIBMYLIB_BASE)/source.c|variant  $(LIBMYLIB_BUILD)/source.variant.o
$(LIBMYLIB_ORIGIN)/source.c|origin  $(LIBMYLIB_BUILD)/source.origin.o
*_[FILENAME]_FLAGS Build flags for source file without variant specification
*_[FILENAME]_INCLUDES Includes for source file without variant specification
*_[FILENAME]_[VARIANT]_FLAGS Build flags for source file for variant [VARIANT]
*_[FILENAME]_[VARIANT]_INCLUDES Includes for source file for variant [VARIANT]

Makefile.uk: Externally Compiled Sources
▌Unikraft supports including externally compiled sources
Cases where it may happen:
Code only available as binary form
Compiling is done by different build system
(e.g., invoked by custom prepare rules)
▌Depending on the type, various places exist to add them:
.o-object files are added to the *_OBJS-y list:
.o-libraries are registered to the global list UK_OLIBS
(remember to use the library switch CONFIG_*):
Static libraries are registered to the global list UK_ALIBS
Note: Shared libraries (.so) are currently not supported
LIBMYLIB_OBJS-y += $(LIBMYLIB_BASE)/prebuilt.o
UK_ALIBS-$(CONIG_LIBMYLIB) += $(LIBMYLIB_BASE)/static_lib.a
UK_OLIBS-$(CONFIG_LIBMYLIB) += $(LIBMYLIB_BASE)/prebuilt_lib.o

Makefile.uk: Scope of Headers (Includes)
▌Global headers (e.g., library API)
▌Library-Internal headers
▌File-specific headers
▌Equivalent to this, you can set build flags within a specific scope
CFLAGS-$(CONFIG_LIBMYLIB), LIBMYLIB_CFLAGS-y,
LIBMYLIB_MYSRC_FLAGS, LIBMYLIB_MYSRC_VARIANT_FLAGS
CINCLUDES-$(CONFIG_LIBMYLIB) += -I$(LIBMYLIB_BASE)/include/api
LIBMYLIB_CINCLUDES-y += -I$(LIBMYLIB_BASE)/include/internal
# mysrc.c
LIBMYLIB_MYSRC_INCLUDES += -I$(LIBMYLIB_BASE)/include/mysrc
# Variant var0 of mysrc.c: mysrc.c|var0
LIBMYLIB_MYSRC_VAR0_INCLUDES += -I$(LIBMYLIB_BASE)/include/mysrc

Makefile.uk: Custom Prepare Rules
▌Reason
Generate files (headers, sources) needed for build
Invoke parts of ported library build system, like ./configure
▌Defined as custom Make rules
Use build_cmd to prettify the output1
(in cases where build_cmd is not applicable use verbose_cmd)
▌If used, set marker of fetch stage as dependency
Download marker: $(LIBMYLIB_BUILD)/.origin
Patched marker: $(LIBMYLIB_BUILD)/.patched
▌Register generated files to prepare stage
UK_PREPARE-$(CONFIG_LIBMYLIB) += [generated file/phony]
$(LIBMYLIB_BUILD)/generated.h: [dependencies]
$(call build_cmd,NM,libmylib,$@,$(NM) -n $(LIBMYLIB_BASE)/symtab.in > $@)

exportsyms.uk
▌Re-masks the scope of each symbol of a library
 Re-defines for each symbol if it is available for final linking
 Intended to reduce potential clashing of symbols
▌List of symbol names that should be available globally for final
linking. Non-listed symbols become private to the library.
 Example (libnolibc):
▌Note: The build system will throw a warning when no exportsyms.uk
file is provided
 The scope of each library symbol stays unchanged in such a case
asprintf
vasprintf
# comments are ignored
opterr
optind
optopt
optreset
optarg
getopt
[...]

Best Practices
▌Porting existing libraries/applications is a challenging task
 Existing sources often only fit to their build and configuration system
 Often not intended to run on something else than Linux (assumptions to the OS)
▌If possible, compile all sources with Unikraft
 Including external build binaries is risky
• Build flags may be incompatible (e.g., register usage/calling convention, LTO)
• Mismatch of depending libraries (external vs. Unikraft‘s version)
▌Learn from existing build system
 Extract list of source files and build flags when compiling with original build system
 Study steps that generate files needed for the build
• Try to run ./configure with settings fitting to Unikraft environment
• It is also possible to call ./configure from Unikraft as prepare step
▌Provide initial stubs for missing symbols
 Completing compiling & linking (but not running) first,
helps to get an better overview of missing functionality in Unikraft

XPDDS19: A Journey through Unikraft's Build System

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