c - How does the user-mode kernel in UML interface with the underlying kernel on the host -


in user mode linux (uml) trace thread annuls system calls made user-space process , redirects them kernel running in user-space. @ point userspace kernel require assistance of host os. how user-mode kernel call underlying host kernel, normal system calls? or use kind of ioctl mechanism or else?

i found simple explanation of uml design here. may useful you.

uml constructs using ptrace system call tracing mechanism.

  1. when process in user space, system called intercepted ptrace.
  2. when in kernel, no interception. when process executes system call or receives signal, tracing thread forces process run in kernel.
  3. after transition, process state restored , continues.

system call virtualization

  1. by switching user , kernel , system calls interception
  2. note: system call must annulled in host kernel.
  3. the process state preserved.
  4. when system call complete, process obtains returned value in saved registers , returned user mode.

also, article explains system call virtualization using ptrace following picture.

http://www.csee.wvu.edu/~katta/uml/graphics/vsyscall.jpg. red line represents normal syscall procedure.


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