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Authors: Brandon Potter

===============================================================================

This file exists to educate users and notify them that some filesystem open
system calls may have been redirected by system call emulation mode
(henceforth se-mode).

To provide background, system calls to open files with SYS_OPEN (man 2 open)
inside se-mode will resolve by pass-through to glibc calls (man 3 open) on the
host machine. The host machine will open the file on behalf of the simulator.
Subsequently, se-mode acts as a shim for file access to the opened file. By
utilizing the host machine, se-mode gains quite a bit of utility without
needing to implement an actual filesystem.

A scenario for using normal files might be `/bin/cat $HOME/my_data_file`
as the simulated application (and option). The simulator leverages the host
file system to provide access to my_data_file in this case. Several things
happen inside the simulator:
    1) The cat command will open $HOME/my_data_file by invoking the open
system call (SYS_OPEN). In se-mode, SYS_OPEN is trapped by the simulator and
the syscall_emul.hh:openImpl implementation is provided as a drop-in
replacement for what normally occurs inside a real operating system.
    2) The openImpl code will pass through several path checks and realize
that the file needs to be handled in the 'normal' case where se-mode utilizes
the host filesystem.
    3) The openImpl code will use the glibc open library call on
$HOME/my_data_file after normalizing invocation options.
    4) If the file successfully opens, se-mode will record the file descriptor
returned from the glibc open and provide a translated file descriptor to the
application. (If the glibc's file descriptor was passed back to the
application, it would be noticable that the application runtime environment
was wonky. The gem5.{opt,debug,fast} process needs to open files for its own
purposes and the file descriptors for the simulated application perspective
would appear out-of-order and arbitrary. They should appear in-order with the
lowest available file-desciptor assigned on calls to SYS_OPEN. So, se-mode
adds a level of indirection to resolve this problem.)

However, there are files which users might not want to open on the host
machine; providing file access and/or file visibility to the simulated
application may not make sense in these cases. Historically, these files
have been handled by os-specific code in se-mode. The os-specific
implementation has been referred to as 'special files'. Examples of
special file implementations include /proc/meminfo and /etc/passwd. (See
src/kern/linux/linux.cc for more details.)

A scenario for using special files might be running `/bin/cat /proc/meminfo`
as the simulated application (and option). Several things will happen inside
the simulator:
    1) The cat command will open the /proc/meminfo file by invoking the open
system call (SYS_OPEN). In se-mode, SYS_OPEN is trapped by the simulator and
the syscall_emul.hh:openImpl implementation is provided as a drop-in
replacement for what normally occurs inside a real operating system.
    2) The openImpl code checks to see if /proc/meminfo matches a special
file. When it notices the match, it invokes code to generate a replacement
file rather than open the file on the host machine. (As it turns out, opening
the host's version of /proc/meminfo will resolve to the gem5 executable which
is probably not what the application intended.)
    3) The generated file is provided a file descriptor (which itself has
special handling to preserve the illusion that the application is not running
inside a simulator under weird conditions). The file descriptor is passed
back to the application and it can subsequently use the file descriptor to
access the redirected /proc/meminfo file.

Regarding special files, a subtle but important point is that these files
are generated dynamically during simulation (in C++ code). Certain files,
such as /proc/meminfo depend on the application state inside the simulator to
have valid contents. With some files, you generally cannot anticipate what
file contents should be before the application actually tries to inspect the
contents. These types of files should all be handled using the special files
method.

As an aside, users might also want to restrict the contents of a file to
prevent non-determinism in the simulation. (This is another case for special
handling of files.) It can be annoying to try to generate statistics for your
new hardware widget (which of course will improve performance by some
non-trivial percentage) when variance in the statistics is caused by
randomness of file contents. A specific example which comes to mind is
reading the contents of /dev/random. Ideally, se-mode should introduce no
non-determinism. However, that is difficult (if not impossible) to achieve in
practice for every application thrown at the simulator.

In addition to special files, there is another method to handle filesystem
redirection. Instead of dynamically generating a file and providing it to
the application, it is possible to pregenerate files on the host filesystem
and redirect open calls to the pregenerated files. This is achieved by
capturing the paths provided by the application SYS_OPEN and modifying the
path before issuing the pass-through call to the host filesystem glibc open.
The name for this feature is 'faux filesystem' (henceforth faux-fs).

With faux-fs, users can add paths via command line (via --chroot) or by
modifying their configuration file to use the RedirectPath class. These
paths take the form of original_path-->set_of_modified_paths. For instance,
/proc/cpuinfo might be redirected to /usr/local/gem5_fs/cpuinfo __OR__
/home/me/gem5_folder/cpuinfo __OR__ /nonsensical_name/foo_bar, etc.. The
matching pattern and directory/file-structure is controlled by the user. The
pattern match hits on the first available file which actually exists on the
host machine.

As another subtle point, the faux-fs handling is fixed at simulator
configuration time. The path redirection becomes static after configuration
and the Python generated files in simout/fs/.. also exist after configuration.
The faux-fs mechanism is __NOT__ suitable for files such a /proc/meminfo
since those types of files rely on runtime application characteristics.

Currently, faux-fs is setup to create a few files on behalf of the average
user. These files are all stuffed into the simout directory under a 'fs'
folder. By default, the path is $gem5_dir/m5out/fs. These files are all
hardcoded in the configuration since it is unlikely that an application wants
to see the host version of the files. At the time of writing, the list can be
viewed in configs/example/se.py by searching for RedirectPath. Most of
the faux-fs Python generated files depend on simulator configuration (i.e.
number of cores, caches, nodes, etc..). Sophisiticated runtimes might query
these files for hardware information in certain applications (i.e.
applications using MPI or ROCm since these runtimes utilize libnuma.so).

Of note, dynamically executables will open shared object files in the same
manner as normal files. It is possible and maybe enen preferential to utilize
the faux-fs to create a platform independent way of running applications in
se-mode. Users can stuff all the shared libraries into a folder and commit the
folder as part of their repository state. The chroot option can be made to
point to the shared library folder (for each library) and these libraries will
be redirected away from host libraries. This can help to alleviate environment
problems between machines.

If there is any confusion on path redirection, the system call debug traces
can be used to emit information regarding path redirection.