vDSOs (virtual dynamically linked shared objects) are a Linux kernel mechanism for exporting a carefully selected set of kernel space routines to user space applications so that applications can call these kernel space routines in-process, without incurring the performance penalty of a context switch that is inherent when calling these same kernel space routines by means of the system call interface.
vDSO uses standard mechanisms for linking and loading i.e. standard Executable and Linkable Format (ELF) format.
It helps to reduce the calling overhead on simple kernel routines, and also can work as a way to select the best system call method on some architectures.
An advantage over other methods is that such exported routines can provide proper DWARF (Debug With Attributed Record Format) debugging information.
Implementation generally implies hooks in the dynamic linker to find the vDSOs.