Libunwind with Patches for Xen Unikernel Unwinding

(Note: the original, upstream readme can be found in README.)

This is a fork of libunwind (http://www.nongnu.org/libunwind/), a library to unwind the stack and create a callchain. This fork is extended by support for Xen virtual machines. This is especially useful for Unikernels, in which the kernel and application form a single binary sharing a common address space. By using libunwind-xen, a unikernel's (or, in theory, any other VM's) stack can be unwound from the outside. One of the main uses for this is the sampling of call chains at runtime as a profiling tool.

The way libunwind does this is by using the .eh_frame information created by a compiler at compile time. By comparing the stack addresses in the running VM to those in the ELF's .eh_frame section, it can assess the size of the currently running function's frame on the stack, unroll it to retrieve the prveious instruction pointer, and iterate over this until the end of the stack is reached. This means that you need compiler support to create an .eh_frame, but the good news is that this is almost universally supported out-of-the-box and by default. It also means that you do not need a frame pointer, so you can produce stack traces on binaries compiles with -fomit-frame-pointer for higher performance, or on architectures that do not provide a frame pointer register.

This current implementation is quite bare-bones and does not support functionality that isn't required for stack walking. For example, there is no support for writing memory or registers, only reading. It currently also only supports the x86 version of Xen. Extensions of this starting point should be easy though, if more is required at any point in the future.

Build and Install

1. ./autogen.sh
2. ./configure
3. make
4. make install

Use

For tracing Xen virtual machines, link your application against libunwind-generic and libunwind-xen.