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In gem5::RiscvISA::ISA, handleLocked* functions maintain an address stack(i.e. locked_addrs) to check whether each SC matches the most recent LR. However, there are some problems with this implementation. First, the elements in the stack may only be popped when the handleLockedSnoop function is invoked. In other cases, the elements in the stack will not be popped even if the SC and LR match. This makes the `locked_addrs` get bigger and bigger as gem5 runs. Second, LR/SC does not always match. For example, in Linux's __cmpxchg[1], after executing LR, if the value read is not equal to the old value, the subsequent SC is skipped. For gem5's current implementation, this would cause the address to be pushed into `locked_addrs` every time __cmpxchg is failed. But these addresses are never popped. This also makes the `locked_addrs` get bigger and bigger. Third, existing emulator implementations (spike, qemu) do not use the stack, but only record the last address accessed by LR. Afterward, when executing SC, these implementations determine whether the address accessed by SC is the same as the one recorded. This patch modifies gem5's handleLocked* function by referring to other existing RISC-V implementations. It eliminates `locked_addrs` and simplifies the related code. Thus, it fixes the "memory leak"-like error that can occur on `locked_addrs` when executing LR/SC. Related links: [1] Linux's cmpxchg implementation for RISC-V: + https://github.com/torvalds/linux/blob/master/arch/riscv/include/asm/cmpxchg.h [2] spike lr/sc implementation: + https://github.com/riscv-software-src/riscv-isa-sim/blob/master/riscv/insns/sc_d.h + https://github.com/riscv-software-src/riscv-isa-sim/blob/master/riscv/insns/lr_d.h + https://github.com/riscv-software-src/riscv-isa-sim/blob/master/riscv/mmu.h [3] rocket lr/sc implementation: + https://github.com/chipsalliance/rocket-chip/blob/master/src/main/scala/rocket/NBDcache.scala [4] QEMU lr/sc implementation: + https://gitlab.com/qemu-project/qemu/-/blob/master/target/riscv/insn_trans/trans_rva.c.inc Change-Id: Ic79444cace62e39b7fe9e01f665cb13e4d990d0a Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/55663 Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu> Reviewed-by: Jason Lowe-Power <power.jg@gmail.com> Maintainer: Jason Lowe-Power <power.jg@gmail.com> Tested-by: kokoro <noreply+kokoro@google.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, 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 these tools. Once you have all dependencies resolved, type 'scons build/<CONFIG>/gem5.opt' where CONFIG is one of the options in build_opts like ARM, NULL, MIPS, POWER, SPARC, X86, Garnet_standalone, etc. This will build an optimized version of the gem5 binary (gem5.opt) with the the specified configuration. See http://www.gem5.org/documentation/general_docs/building for more details and options. The main source tree includes these subdirectories: - build_opts: pre-made default configurations for gem5 - build_tools: tools used internally by gem5's build process. - configs: example simulation configuration scripts - ext: less-common external packages needed to build gem5 - include: include files for use in other programs - site_scons: modular components of the build system - 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 may need compiled system firmware, kernel binaries and one or more disk images, depending on gem5's configuration and what type of workload you're trying to run. Many of those resources can be downloaded from http://resources.gem5.org, and/or from the git repository here: https://gem5.googlesource.com/public/gem5-resources/ If you have questions, please send mail to gem5-users@gem5.org Enjoy using gem5 and please share your modifications and extensions.