base: Add a class which encapsulates Fibers.

This class encapsulates the idea of a Fiber in such a way that other
implementations can be substituted in in the future. This
implementation uses the ucontext family of functions.

This change also adds a new unit test which exercises the new class. It
creates three new fibers which accept a sequence of other fibers to
switch to, one after the other. The main test function switches to
the these fibers which switch with each other and occasionally back to
the main fiber. Each time a test fiber is activated, it checks against
a list which shows the correct order for the fibers to run in. When the
main fiber gets control, it makes sure that list has been progressed
through by the correct amount.

Change-Id: I1fc2afa414b51baaa91e350a4ebc791d989f0b8a
Reviewed-on: https://gem5-review.googlesource.com/10935
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>

4 files changed, 396 insertions, 0 deletions

diff --git a/src/base/SConscript b/src/base/SConscript
index a90b78486..b3205a6bb 100644
--- a/src/base/SConscript
+++ b/src/base/SConscript
@@ -46,6 +46,8 @@ if env['USE_FENV']:
     Source('fenv.c')
 if env['USE_PNG']:
     Source('pngwriter.cc')
+Source('fiber.cc')
+GTest('fibertest', 'fibertest.cc', 'fiber.cc')
 Source('framebuffer.cc')
 Source('hostinfo.cc')
 Source('inet.cc')
diff --git a/src/base/fiber.cc b/src/base/fiber.cc
new file mode 100644
index 000000000..f10f1fbfd
--- /dev/null
+++ b/src/base/fiber.cc
@@ -0,0 +1,148 @@
+/*
+ * Copyright 2018 Google, Inc.
+ *
+ * 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: Gabe Black
+ */
+
+#include "base/fiber.hh"
+
+#include <cerrno>
+#include <cstring>
+
+#include "base/logging.hh"
+
+using namespace std;
+
+namespace
+{
+
+/*
+ * The PrimaryFiber class is a special case that attaches to the currently
+ * executing context. That makes handling the "primary" fiber, aka the one
+ * which most of gem5 is running under, no different than other Fibers.
+ */
+class PrimaryFiber : public Fiber
+{
+  public:
+    PrimaryFiber() : Fiber(nullptr, 0) { setStarted(); }
+    void main() { panic("PrimaryFiber main executed.\n"); }
+};
+
+PrimaryFiber _primaryFiber;
+
+// A pointer to whatever the currently executing Fiber is.
+Fiber *_currentFiber = &_primaryFiber;
+
+// A pointer to the Fiber which is currently being started/initialized.
+Fiber *startingFiber = nullptr;
+
+} // anonymous namespace
+
+void
+Fiber::entryTrampoline()
+{
+    startingFiber->start();
+}
+
+Fiber::Fiber(size_t stack_size) :
+    link(primaryFiber()),
+    stack(stack_size ? new uint8_t[stack_size] : nullptr),
+    stackSize(stack_size), started(false), _finished(false)
+{}
+
+Fiber::Fiber(Fiber *link, size_t stack_size) :
+    link(link), stack(stack_size ? new uint8_t[stack_size] : nullptr),
+    stackSize(stack_size), started(false), _finished(false)
+{}
+
+Fiber::~Fiber()
+{
+    panic_if(stack && _currentFiber == this, "Fiber stack is in use.");
+    delete [] stack;
+}
+
+void
+Fiber::createContext()
+{
+    // Set up a context for the new fiber, starting it in the trampoline.
+    getcontext(&ctx);
+    ctx.uc_stack.ss_sp = stack;
+    ctx.uc_stack.ss_size = stackSize;
+    ctx.uc_link = nullptr;
+    makecontext(&ctx, &entryTrampoline, 0);
+
+    // Swap to the new context so it can enter its start() function. It
+    // will then swap itself back out and return here.
+    startingFiber = this;
+    panic_if(!_currentFiber, "No active Fiber object.");
+    swapcontext(&_currentFiber->ctx, &ctx);
+
+    // The new context is now ready and about to call main().
+}
+
+void
+Fiber::start()
+{
+    // Avoid a dangling pointer.
+    startingFiber = nullptr;
+
+    setStarted();
+
+    // Swap back to the parent context which is still considered "current",
+    // now that we're ready to go.
+    int ret M5_VAR_USED = swapcontext(&ctx, &_currentFiber->ctx);
+    panic_if(ret == -1, strerror(errno));
+
+    // Call main() when we're been reactivated for the first time.
+    main();
+
+    // main has returned, so this Fiber has finished. Switch to the "link"
+    // Fiber.
+    _finished = true;
+    link->run();
+}
+
+void
+Fiber::run()
+{
+    panic_if(_finished, "Fiber has already run to completion.");
+
+    // If we're already running this fiber, we're done.
+    if (_currentFiber == this)
+        return;
+
+    if (!started)
+        createContext();
+
+    // Switch out of the current Fiber's context and this one's in.
+    Fiber *prev = _currentFiber;
+    Fiber *next = this;
+    _currentFiber = next;
+    swapcontext(&prev->ctx, &next->ctx);
+}
+
+Fiber *Fiber::currentFiber() { return _currentFiber; }
+Fiber *Fiber::primaryFiber() { return &_primaryFiber; }
diff --git a/src/base/fiber.hh b/src/base/fiber.hh
new file mode 100644
index 000000000..b9f0683a5
--- /dev/null
+++ b/src/base/fiber.hh
@@ -0,0 +1,106 @@
+/*
+ * Copyright 2018 Google, Inc.
+ *
+ * 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: Gabe Black
+ */
+
+#ifndef __BASE_FIBER_HH__
+#define __BASE_FIBER_HH__
+
+#include <ucontext.h>
+
+#include <cstddef>
+#include <cstdint>
+
+/**
+ * This class represents a fiber, which is a light weight sort of thread which
+ * is cooperatively scheduled and runs sequentially with other fibers, swapping
+ * in and out of a single actual thread of execution.
+ *
+ * To define your own threads, create a subclass of Fiber and override its
+ * main() function to do what you want your fiber to do. You can start it by
+ * calling its run() method which will stop your execution and start the other
+ * fiber in your place.
+ *
+ * If your main() function ends, that fiber will automatically switch to either
+ * the primary fiber, or to a particular fiber you specified at construction
+ * time, and your fiber is considered finished.
+ */
+
+class Fiber
+{
+  public:
+    const static size_t DefaultStackSize = 0x50000;
+
+    /// stack_size is the size of the stack available to this fiber.
+    /// link points to another fiber which will start executing when this
+    /// fiber's main function returns.
+    Fiber(size_t stack_size=DefaultStackSize);
+    Fiber(Fiber *link, size_t stack_size=DefaultStackSize);
+
+    virtual ~Fiber();
+
+    /// Start executing the fiber represented by this object. This function
+    /// will "return" when the current fiber is switched back to later on.
+    void run();
+
+    /// Returns whether the "main" function of this fiber has finished.
+    ///
+    bool finished() const { return _finished; };
+
+    /// Get a pointer to the current running Fiber.
+    ///
+    static Fiber *currentFiber();
+    /// Get a pointer to the primary Fiber.
+    /// This Fiber represents the thread of execution started by the OS, and
+    /// which has a Fiber attached to it after the fact.
+    static Fiber *primaryFiber();
+
+  protected:
+    /// This method is called when this fiber is first run. Override it to
+    /// give your fiber something to do. When main returns, the fiber will
+    /// mark itself as finished and switch to its link fiber.
+    virtual void main() = 0;
+
+    void setStarted() { started = true; }
+
+  private:
+    static void entryTrampoline();
+    void start();
+
+    ucontext_t ctx;
+    Fiber *link;
+
+    // The stack for this context, or a nullptr if allocated elsewhere.
+    uint8_t *stack;
+    size_t stackSize;
+
+    bool started;
+    bool _finished;
+    void createContext();
+};
+
+#endif // __BASE_FIBER_HH__
diff --git a/src/base/fibertest.cc b/src/base/fibertest.cc
new file mode 100644
index 000000000..5ed13d425
--- /dev/null
+++ b/src/base/fibertest.cc
@@ -0,0 +1,140 @@
+/*
+ * Copyright 2018 Google, Inc.
+ *
+ * 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: Gabe Black
+ */
+
+#include <gtest/gtest.h>
+
+#include <initializer_list>
+#include <iostream>
+#include <vector>
+
+#include "base/fiber.hh"
+
+class TestFiber : public Fiber
+{
+  public:
+    const char *name;
+    std::vector<Fiber *> next;
+
+    TestFiber(const char *name, std::initializer_list<Fiber *> l);
+
+    void checkExpected();
+    void main();
+};
+
+extern TestFiber a;
+extern TestFiber b;
+extern TestFiber c;
+
+TestFiber a("A", { &b, &a, Fiber::primaryFiber(), &b, &c });
+TestFiber b("B", { &a, &c });
+TestFiber c("C", { &a, Fiber::primaryFiber(), Fiber::primaryFiber() });
+
+std::vector<TestFiber *>::iterator expectedIt;
+std::vector<TestFiber *> expected({
+    &a, &b, &a, &a, /* main Fiber, */
+    &a, &b, &c, &a, &c,
+    /* main Fiber, */ &c, &c
+});
+
+TestFiber::TestFiber(
+        const char *name, std::initializer_list<Fiber *> l) :
+    name(name), next(l)
+{}
+
+void
+TestFiber::checkExpected()
+{
+    ASSERT_NE(expectedIt, expected.end());
+    TestFiber *e = *expectedIt++;
+    EXPECT_EQ(e, this) << "Expected " << e->name << ", got " << name;
+}
+
+void
+TestFiber::main()
+{
+    checkExpected();
+    for (auto &n : next) {
+        n->run();
+        checkExpected();
+    }
+}
+
+TEST(Fiber, Switching)
+{
+    expectedIt = expected.begin();
+
+    a.run();
+    EXPECT_EQ(expectedIt - expected.begin(), 4);
+
+    a.run();
+    EXPECT_EQ(expectedIt - expected.begin(), 9);
+
+    c.run();
+    EXPECT_EQ(expectedIt - expected.begin(), 10);
+
+    EXPECT_FALSE(a.finished());
+    EXPECT_FALSE(b.finished());
+    EXPECT_FALSE(c.finished());
+
+    c.run();
+    EXPECT_EQ(expected.end(), expectedIt) <<
+        "Didn't exactly use up the expected Fiber sequence";
+
+    EXPECT_TRUE(c.finished());
+}
+
+int currentIndex = 0;
+
+class LinkedFiber : public Fiber
+{
+  public:
+    const int index;
+    LinkedFiber(Fiber *link, int index) : Fiber(link), index(index) {}
+
+    void
+    main()
+    {
+        EXPECT_EQ(currentIndex, index);
+        currentIndex++;
+    }
+};
+
+TEST(Fiber, Linked)
+{
+    currentIndex = 0;
+
+    LinkedFiber lf3(Fiber::primaryFiber(), 3);
+    LinkedFiber lf2(&lf3, 2);
+    LinkedFiber lf1(&lf2, 1);
+    LinkedFiber lf0(&lf1, 0);
+
+    lf0.run();
+
+    EXPECT_EQ(currentIndex, 4);
+}