sim-se: add a faux-filesystem

This change introduces the concept of a faux-filesystem.
The faux-filesystem creates a directory structure in m5out
(or whatever output dir the user specifies) where system calls
may be redirected.

This is useful to avoid non-determinism when reading files
with varying path names (e.g., variations from run-to-run if
the simulation is scheduled on a cluster where paths may change).

Also, this changeset allows circumventing host pseudofiles which
have information specific to the host processor (such as cache
hierarchy or processor information). Bypassing host pseudofiles
can be useful when executing runtimes in the absence of an
operating system kernel since runtimes may try to query standard
files (i.e. /proc or /sys) which are not relevant to an
application executing in syscall emulation mode.

Change-Id: I90821b3b403168b904a662fa98b85def1628621c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/12119
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/sim/redirect_path.hh

@@ -0,0 +1,65 @@
+/*
+ * Copyright (c) 2015 Advanced Micro Devices, Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Authors: David Hashe
+ */
+
+#ifndef __SIM_REDIRECT_PATH_HH__
+#define __SIM_REDIRECT_PATH_HH__
+
+#include <string>
+#include <vector>
+
+#include "params/RedirectPath.hh"
+#include "sim/sim_object.hh"
+
+/**
+ * RedirectPath stores a mapping from one 'appPath' to a vector of
+ * 'hostPath'. Each 'appPath' and 'hostPath' is a filesystem path.
+ * Used by process.cc to redirect syscall accesses to different directories.
+ */
+class RedirectPath : public SimObject
+{
+  public:
+    RedirectPath(const RedirectPathParams *p);
+
+    const std::string& appPath() { return _appPath; };
+    const std::vector<std::string>& hostPaths() { return _hostPaths; };
+
+  protected:
+    /**
+     * An appPath is a path which needs to be redirected and replaced
+     * by one of the corresponding hostPath (when accessing files on the host
+     * filesystem.)
+     */
+     // _appPath holds the path as it would appear from an app's perspective.
+    std::string _appPath;
+     // _hostPaths holds a set of host filesystem paths
+    std::vector<std::string> _hostPaths;
+};
+
+#endif