What are main issues while porting VxWorks code to Linux?

Moderately hard.

Yes.

I googled "Linux interrupt routines" and a large number of hits came up. Why can't you do the same ?

"Hi all, VxWorks-to-Linux migration kits are offered by a number of companies, including MapuSoft, LynuxWorks, MontaVista, and TimeSys."

Porting VxWorks Applications to Linux

MontaVista Offers Easy VxWorks-To-Linux Transition; VxWorks 'virtual machine' runs legacy embedded code on Linux platform

MontaVista aims tool at the heart of market leader, Wind River

"The new VxWorks-to-Linux tool kit presents applications with a VxWorks "virtual machine" that emulates key system calls and behaviors of the VxWorks kernel on a standard Linux platform. Basically, a library of translations converts VxWorks application program interfaces (APIs) into functions that are compatible with Linux. MontaVista claims that source code utilizing VxWorks APIs can often be recompiled and run on Linux after little or no modification, other than minor changes to header files. According to Ready, "programmers should be able to convert VxWorks-based applications into ones that run on Hard Hat Linux in a handful of weeks." Emulating and/or translating all of the approximately 1500 API (application program interface) functions of VxWorks would have presented a severe challenge. However, MontaVista developers determined that it would be sufficient to limit their focus to roughly fifty of the most commonly used VxWorks APIs."

Legacy2Linux - Porting Legacy RTOS Applications

"Legacy2Linux project provides emulation environments which implement most of the core features of selected legacy RTOS kernels on a standard Linux platform. The 'virtual machines' provided by these emulation environments allow companies to test their existing legacy code in an emulation environment, and provide a basis upon which to build successful Linux ports of these applications." There are kits available for both Wind River pSOS (R) and Wind River VxWorks (R).

RTAI has a VxWorks skin, that should take more or less unmodified VxWorks code and compile it. I have not tried this myself, but take alook at rtai.org (RTAI HP) or rtai.dk (RTAI wiki).