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When the system call happens during the execution of the system call instruction, it can be ambiguous what state takes precedence, the state update from the instruction or the system call. These may be tracked differently and found in an unpredictable order in, for example, the O3 CPU. An instruction can avoid updating any state explicitly, but implicitly updated state (specifically the PC) will always update, whether the instruction wants it to or not. If the system call can be deferred by using a Fault object, then it's no longer ambiguous. The PC update will be discarded, and the system call can set the PC however it likes. Because there is no implicit PC update, the PC needs to be walked forward, either to what it would have been anyway, or to what the system call set in NPC. In addition, because of the existing semantics around handling Faults, the instruction no longer needs to be marked as serializing, non-speculative, etc. The "normal", aka architectural, aka FS version of the system call instructions don't return a Fault artificially. Change-Id: I72011a16a89332b1dcfb01c79f2f0d75c55ab773 Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33281 Maintainer: Gabe Black <gabeblack@google.com> Tested-by: kokoro <noreply+kokoro@google.com> Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
This is the gem5 simulator. The main website can be found at http://www.gem5.org A good starting point is http://www.gem5.org/about, and for more information about building the simulator and getting started please see http://www.gem5.org/documentation and http://www.gem5.org/documentation/learning_gem5/introduction. To build gem5, you will need the following software: g++ or clang, Python (gem5 links in the Python interpreter), SCons, SWIG, zlib, m4, and lastly protobuf if you want trace capture and playback support. Please see http://www.gem5.org/documentation/general_docs/building for more details concerning the minimum versions of the aforementioned tools. Once you have all dependencies resolved, type 'scons build/<ARCH>/gem5.opt' where ARCH is one of ARM, NULL, MIPS, POWER, SPARC, or X86. This will build an optimized version of the gem5 binary (gem5.opt) for the the specified architecture. See http://www.gem5.org/documentation/general_docs/building for more details and options. The basic source release includes these subdirectories: - configs: example simulation configuration scripts - ext: less-common external packages needed to build gem5 - src: source code of the gem5 simulator - system: source for some optional system software for simulated systems - tests: regression tests - util: useful utility programs and files To run full-system simulations, you will need compiled system firmware (console and PALcode for Alpha), kernel binaries and one or more disk images. If you have questions, please send mail to gem5-users@gem5.org Enjoy using gem5 and please share your modifications and extensions.