CSE 221: Homework 2

Winter 2013

Due: Thursday, February 21, 2013 at 8am in class

1. The Hoare Monitors paper describes assertions before and after the operations wait and signal in terms of the monitor invariant I and condition B.

   (a) Describe similar assertions for wait and signal using Mesa monitors and briefly explain why.

   (b) The Mesa Monitors paper describes how programmers should test and wait for conditions using Mesa monitors:

   ...
   [1]
   while ( ... )
   {
      [2]
      ...
      [3] b.wait [4]
      ...
   }
   [5]
   ...
   

   Notes:
   • "..." means that arbitrary code could be here
   • [1] is immediately before the while loop starts
   • [2] is at the start of the body of the while loop
   • [3] is immediately before the wait, and [4] immediately after
   • [5] is immediately after the while loop

   Considering the code snippet above, create a table with five rows corresponding to the five placeholders in the code and two columns corresponding to the invariant I and condition B. For each entry in the table, write "hold", "!hold", or "unknown" depending upon what can be assumed about the invariant or condition, respectively, at each point in the code snippet.

2. Keeping secrets

   Butler Lampson once gave a set of principles for system design. Among these, he gave two conflicting pieces of advice on the nature of implementations. He said, "Keep secrets of the implementation. Secrets are assumptions about an implementation that client programs are not allowed to make... Obviously, it is easier to program and modify a system if its parts make fewer assumptions about each other." And yet, "One way to improve performance is to increase the number of assumptions that one part of a system makes about another; the additional assumptions often allow less work to be done, sometimes a lot less." That is, on the one hand we should hide an implementation for ease of development, and, on the other, we should expose our implementations for speed. For example, Xen's paravirtualized x86 interface exposed the implementation of the hypervisor to its guest OSes, largely for speed, while Sprite's process migration took advantage of secrets in hiding the distributed nature of its implementation from processes subject to migration.

   GMS and Exokernel are two more systems which exemplified this advice. For these two systems, explain:

   1. Which advice of Lampson's did the authors follow? That is, describe the service that was implemented, and whether the authors chose to hide or expose in the implementation.
   2. Describe what was hidden or exposed in the implementation, and the software mechanisms that were used to do the hiding or exposing. Be specific.
   3. Give a concrete example of how the mechanisms above were used to hide or expose in the system.
   4. Describe one problem the authors had in utilizing their mechanism for their respective purpose, and how the authors dealt with that problem. Be specific.
   5. Given the quotes above, discuss the authors' goals in following the design principle they chose. Did they achieve them? Justify your answer.

3. Exokernel and L4 represent contemporary approaches for providing protection and extensibility. Xen represents a contemporary approach for providing virtualization and isolation (or, alternately, is an extreme version of extensibility since it goes even beyond Exokernel in exposing the hardware interface to unprivileged code). Consider a Web server as a motivating application-level service running on each of these three system structures, each hosting the OS described in the paper.

   For each of the three systems, consider the path a network packet containing an HTTP request takes as it travels from the network interface card to a Web server process running at user level:

   1. Identify the various protection domains in the system for this scenario. Which domains are privileged, and which are unprivileged? (Feel free to draw "boxes-and-kernel-boundary" diagrams if you find them helpful.)

      For example, if the system were standard monolithic Linux, the protection domains would be the kernel and the Web server process with its address space. The kernel is privileged, and the server process unprivileged.

   2. Describe the journey of the packet as a sequence of steps through the protection domains identified above. For each protection domain crossing, state the communication mechanism used for that packet to cross protection domains.
   3. Argue which of these systems will likely provide the highest performance Web service without violating protection (e.g., not simply moving the Web server code into the kernel and running it in privileged mode). Justify your argument and be sure to state any assumptions you make.
   4. Further consider the Web server process triggering a page fault on a page in its address space. As with the network packet, trace the propagation of the page fault through protection domains. Which domain handles the page fault? Whose pool of physical memory is used to satisfy the page fault?

      For example, if the system were standard monolithic Linux, the CPU would raise an interrupt, halting the Web server process, and vector to a Linux kernel interrupt handler for page faults. The page fault handler would allocate a physical page from Linux's free physical page list and update the page table entry with the valid mapping. The Linux kernel would then return from the interrupt.