base: Provide a getter for Fiber::started boolean variable

This can be used to check if the fiber has started its execution.

Change-Id: Ie9222b8076756363c9f82c1333c76a352bcaf817
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18648
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

base: Fix fiber compilation issue for Mac OS

Mac OS has deprecated the use of ucontext and at the moment we are
using a workaround to enable it. A side-effect is that the code is
marked as _POSIX_C_SOURCE and consequently <sys/mman.h> requires the
code to be also marked as _DARWIN_C_SOURCE to include the definition
of MAP_ANONYMOUS.

Change-Id: I65550d11a0a21cd36d832a7de6320e7e3f332a9d
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/14817
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

base: Set up a guard page for fiber stacks.

This will help detect stack overflow for fibers.

Change-Id: Iff2b102120ec351709e495291d6bead597f8d10c
Reviewed-on: https://gem5-review.googlesource.com/c/14395
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabeblack@google.com>

base: If valgrind is available, tell it about Fiber stacks.

Valgrind can get confused when switching stacks between different
Fibers. If valgrind (and its headers) are available, this change adds
calls to some hooks so valgrind knows where the new stacks are and
doesn't report a bunch of false positives.

Change-Id: I00aefe60372be6de7371dec29427d7182dbee7b6
Reviewed-on: https://gem5-review.googlesource.com/12227
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabeblack@google.com>

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>