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[82.57.177.142]) by smtp.gmail.com with ESMTPSA id p9-20020a1709060dc900b0099bc038eb2bsm9851388eji.58.2023.08.02.21.01.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 02 Aug 2023 21:01:40 -0700 (PDT) From: Andrea Parri To: mathieu.desnoyers@efficios.com, paulmck@kernel.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, Andrea Parri Subject: [RFC PATCH] membarrier: riscv: Provide core serializing command Date: Thu, 3 Aug 2023 06:01:11 +0200 Message-Id: <20230803040111.5101-1-parri.andrea@gmail.com> X-Mailer: git-send-email 2.34.1 MIME-Version: 1.0 X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FROM, RCVD_IN_DNSWL_BLOCKED,SPF_HELO_NONE,SPF_PASS,T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1773189205286505734 X-GMAIL-MSGID: 1773189205286505734 Signed-off-by: Andrea Parri Suggested-by: Palmer Dabbelt Signed-off-by: Andrea Parri --- For the MEMBARRIER maintainers: RISC-V does not have "core serializing instructions", meaning that there is no occurence of such a term in the RISC-V ISA. The discussion and git history about the SYNC_CORE command suggested the implementation below: a FENCE.I instruction "synchronizes the instruction and data streams" [1] on a CPU; in litmus parlance, (single-hart test) CPU0 UPDATE text ; FENCE.I ; EXECUTE text ; /* <-- will execute the updated/new text */ (message-passing test) CPU0 CPU1 UPDATE text | IF (flag) { ; WMB | FENCE.I ; SET flag | EXECUTE text ; /* execute the new text */ | } ; (and many others, including "maybe"s! ;-) ) How do these remarks resonate with the semantics of "a core serializing instruction" (to be issued before returning to user-space)? RISCV maintainers, I'm missing some paths to user-space? (besides xRET) Andrea [1] https://github.com/riscv/riscv-isa-manual/blob/main/src/zifencei.adoc .../sched/membarrier-sync-core/arch-support.txt | 2 +- arch/riscv/Kconfig | 2 ++ arch/riscv/include/asm/sync_core.h | 15 +++++++++++++++ 3 files changed, 18 insertions(+), 1 deletion(-) create mode 100644 arch/riscv/include/asm/sync_core.h diff --git a/Documentation/features/sched/membarrier-sync-core/arch-support.txt b/Documentation/features/sched/membarrier-sync-core/arch-support.txt index 23260ca449468..a17117d76e6d8 100644 --- a/Documentation/features/sched/membarrier-sync-core/arch-support.txt +++ b/Documentation/features/sched/membarrier-sync-core/arch-support.txt @@ -44,7 +44,7 @@ | openrisc: | TODO | | parisc: | TODO | | powerpc: | ok | - | riscv: | TODO | + | riscv: | ok | | s390: | ok | | sh: | TODO | | sparc: | TODO | diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig index 4c07b9189c867..ed7ddaedc692e 100644 --- a/arch/riscv/Kconfig +++ b/arch/riscv/Kconfig @@ -27,6 +27,7 @@ config RISCV select ARCH_HAS_GCOV_PROFILE_ALL select ARCH_HAS_GIGANTIC_PAGE select ARCH_HAS_KCOV + select ARCH_HAS_MEMBARRIER_SYNC_CORE select ARCH_HAS_MMIOWB select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE select ARCH_HAS_PMEM_API @@ -35,6 +36,7 @@ config RISCV select ARCH_HAS_SET_MEMORY if MMU select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL select ARCH_HAS_STRICT_MODULE_RWX if MMU && !XIP_KERNEL + select ARCH_HAS_SYNC_CORE_BEFORE_USERMODE select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST select ARCH_HAS_UBSAN_SANITIZE_ALL select ARCH_HAS_VDSO_DATA diff --git a/arch/riscv/include/asm/sync_core.h b/arch/riscv/include/asm/sync_core.h new file mode 100644 index 0000000000000..d3ec6ac47ac9b --- /dev/null +++ b/arch/riscv/include/asm/sync_core.h @@ -0,0 +1,15 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _ASM_RISCV_SYNC_CORE_H +#define _ASM_RISCV_SYNC_CORE_H + +/* + * Ensure that a core serializing instruction is issued before returning + * to user-mode. RISC-V implements return to user-space through an xRET + * instruction, which is not core serializing. + */ +static inline void sync_core_before_usermode(void) +{ + asm volatile ("fence.i" ::: "memory"); +} + +#endif /* _ASM_RISCV_SYNC_CORE_H */