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Modularization
By design, OSv has always been a "fat" unikernel and provided a large subset of glibc functionality, drivers for many devices, and hypervisors
ZFS is one of the three filesystems that OSv implements. The original code comes from FreeBSD circa 2014 and has been since integrated and adapted to work within OSv. ZFS is one of the best file systems for persistent mutable data and what ones needs to run mysql or elasticsearch on OSv for example. Although a lot of effort has gone into integrating ZFS into OSv, its sheer code base volume makes OSv kernel bigger than it needs to be and there are a plethora of stateless use cases like redis, memcached, microservices, etc where ZFS is not needed and instead we can use ROFS now. ZFS-less kernel is expected to be at least 1MB smaller, which should make OSv images smaller, use less memory, and boot slightly faster. So it would be nice to make it optional as a shared library.
This patch introduces new build mechanism that allows creating custom kernel with specific list of device drivers intended to target given hypervisor. Such kernel benefits from smaller size and better security as all unneeded code is removed.
In essence, we introduce new build script and makefile parameter:
drivers_profile. This new parameter is intended to specify a
drivers profile which is simply a list of device drivers to be linked
into kernel with some extra functionality like PCI or ACPI these drivers
depend on. Each profile is specified in a tiny make include file (.mk)
under new conf/profiles/$(arch) directory and included by the main
makefile as requested by drivers_profile parameter. The main makefile
has number of new ifeq conditions that add given driver object file
to the linked objects list depending on the value (0 or 1) of given
conf_drivers_ variable specified in the relevant profile file.
Sometimes it is necessary to conditionally enable/disable given
code depending on the drivers selected. The good example of it is
arch-setup.cc which actually registers individual drivers and this
is where we need some kind of #if-way of registering given driver.
To that end, this patch adds new script gen-drivers-config-header and
new rule to the makefile, which automatically generates driver-config.h
header file under build/$(mode)/gen/include/osv. The driver-config.h
is comprised of the #define CONF_drivers_* macros that specify if given
driver is enabled or not (1, 0) and is included by relatively few source
file like arch-setup.cc. The extra benefit of this approach is that
every time we change value of drivers_profile, all relevant files are
recompiled and new kernel linked.