Windows Host Quoting Gotcha, and the VProbe Toolkit

Workstation 6.5 is now in public beta. It includes experimental support for VProbes in the guest, virtual machine monitor, and a few spots in the virtual hardware. We'd like to invite anybody curious about vprobes to give it a whirl.

A common problem came up on the forums yesterday: if you're using VProbes from Windows' native command-line, you'll soon notice that the following:

C:\> vmrun vprobeLoad foo.vmx '(vprobe VMM1Hz (printf "greetings windows\n"))'

produces the cryptic error message:

vprobeLoad: error: unknown ident windows\n
vprobeLoad: 0 warnings, 1 errors

What gives? In short, cmd's weird quoting. As far as I can tell, single quotes are basically the same as double-quotes, except cmd will only nest like-with like, so cmd ends up passing along the string '(vprobe VMM1Hz (printf greetings windows\n))'. Check the forum post for the band-aid solution for cmd; I predict you'll only be able to tolerate the syntax for cmd one-liners so long before you give up and install cygwin.

Once you embrace cygwin, you might as well go whole-hog and get the vprobe-toolkit. The abstraction level of vp is very low; while the parentheses might remind you of scheme for a while, pretty soon the semantics will remind you of assembly language. This was an intentional choice: since bugs in the vp compiler can nuke your system, we're motivated to keep it as simple as possible. While it's possible to use vp directly for machine-level things (e.g., instruction or stack profilers), for more complex tasks automatic code generation is more appropriate.

Introducing Emmett

Emmett is a small language that provides C-like type, expression, and conditional operators, some syntactic sugar for aggregation, and automatic inference of type for undeclared variables. The vprobe-toolkit provides a wrapper script, named "vprobe", that invokes the Emmett compiler on your input, loads the resulting VP into the target VM, and runs user-specified pretty-printers on your output. E.g., "vprobe -a" pretty-prints sorted aggregates, and "vprobe -s" pretty-prints stacks.

Where in VP, you'd have to write:

(defaggr a 1 1)
(vprobe USEC:1001
 (do
  (aggr a (VCPUID) ((curprocname)) 1)
  (logaggr a )))

Emmett produces the more readable:

USEC:1001
{
  a[VCPUID, curprocname()]++;
  logaggr(a);      # OK, this is a lot of output.
}

There are a bunch of examples in the cookbook subdirectory.

I want to emphasize that Emmett is a replaceable component. If you're a python programmer, and curly braces make your shoulder blades clench with rage, feel free to write your own little language...

Pragmatics

vprobe-toolkit is open source. We've licensed it with a variant of the BSD license, which VMware's lawyers assure me means you can basically do whatever you want with it; if you're feeling paranoid, though, please run the license file past your local lawfolk rather than taking my word for it. You'll notice the link above is to a subversion repository, not a download page. In its current state, it would be inappropriate to expose it to those unwilling to hack a bit; if ws 6.5 is currently in "beta", this is probably somewhere past "prototype" but not really "alpha" yet. Join us!