Created PNaCl translation documenation and updated old NaCl documents.

services/nacl/README.md

+About
+=====
+
+This directory contains the services required to execute both nexes and pexes.
+
+Using
+=====
+
+For information about how to build and use nexes and pexes from within Mojo,
+refer to the `mojo/nacl` directory.
+
+Non-SFI Nexe Content Handler
+----------------------------
+
+The nexe content handler is simple. It:
+
+1. Acquires the desired nexe and dumps it into a temporary file,
+2. Accesses the MojoHandle connecting the content handler to the shell, and
+3. Launches the Non-SFI nexe.
+
+Non-SFI Pexe Content Handler
+----------------------------
+
+The pexe content handler is slightly more complex. Although it is similar
+in structure to the Non-SFI nexe content handler, it has an additional step
+between item 1 and 2: convert the incoming pexe into a nexe.
+
+This pexe to nexe translation requires two steps: compilation and linking.
+For each of these steps, a helper service is launched. These helper services
+are actually executed as nexes -- `pnacl_llc.nexe` and `ld.nexe`. The
+translation done by these nexes is executed as part of a callback to IRT
+functions, `nacl_irt_private_pnacl_translator_compile`, and
+`nacl_irt_private_pnacl_translator_link`. This makes communication between
+the content handler and these helper nexes more complicated.
+
+For the full picture of the compilation process:
+
+* PexeContentHandler
+  * Create a message pipe, which has a parent and child handle
+  * Call `PexeCompilerStart`, passing in the child end of the message pipe. This
+  contacts a new service which is responsible for launching `pnacl_llc.nexe`.
+* PexeCompiler (new process)
+  * Launch the pnacl_llc nexe using the child end of the pipe received from the
+  PexeContentHandler
+  * The pnacl_llc nexe uses the IRT to make a call to ServeTranslateRequest
+  (defined in `mojo/nacl/nonsfi/irt_pnacl_translator_compile.cc`). This creates
+  the `PexeCompiler` service, which is ready to handle a single request. It is
+  bound to the child end of the pipe.
+* Meanwhile, back in the PexeContentHandler:
+  * Bind the parent end of the pipe to a `PexeCompiler` service. Now,
+  `PexeCompile` can be called with inputs defined by a mojom interface, and
+  outputs can be received via callback
+  * Call `PexeCompile`, passing in the name of the pexe, and receiving the
+  object files created by compilation
+* Meanwhile, back in the pnacl_llc nexe:
+  * Actually do the requested compilation, and pass back the newly created
+  object files
+
+The linking process works similarly, but utilizes a different interface which
+lets it receive object files and return a linked nexe.
+
+Once both the compilation and linking steps have been completed, the
+PexeContentHandler is able to launch the requested nexe.
+
+TODO
+====
+
+* Turn the NexeContentHandler, PexeContentHandler, PexeCompiler, and
+PexeLinker services into "anti-singletons". When each service is requested, a
+unique process must be created.
+* Use subzero (a NaCl project to improve translation speed) for pexe
+translation.
+* Enable caching of translated pexes.