From cd9b29014dc69609489261efe351d0c7709ae8bf Mon Sep 17 00:00:00 2001 From: Andrea Parri Date: Wed, 31 Jan 2024 15:49:36 +0100 Subject: [PATCH] membarrier: riscv: Provide core serializing command RISC-V uses xRET instructions on return from interrupt and to go back to user-space; the xRET instruction is not core serializing. Use FENCE.I for providing core serialization as follows: - by calling sync_core_before_usermode() on return from interrupt (cf. ipi_sync_core()), - via switch_mm() and sync_core_before_usermode() (respectively, for uthread->uthread and kthread->uthread transitions) before returning to user-space. On RISC-V, the serialization in switch_mm() is activated by resetting the icache_stale_mask of the mm at prepare_sync_core_cmd(). Suggested-by: Palmer Dabbelt Signed-off-by: Andrea Parri Reviewed-by: Mathieu Desnoyers Link: https://lore.kernel.org/r/20240131144936.29190-5-parri.andrea@gmail.com Signed-off-by: Palmer Dabbelt --- .../membarrier-sync-core/arch-support.txt | 18 +++++++++++- MAINTAINERS | 1 + arch/riscv/Kconfig | 3 ++ arch/riscv/include/asm/membarrier.h | 19 ++++++++++++ arch/riscv/include/asm/sync_core.h | 29 +++++++++++++++++++ kernel/sched/core.c | 4 +++ kernel/sched/membarrier.c | 4 +++ 7 files changed, 77 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 d96b778b87ed..7425d2b994a3 100644 --- a/Documentation/features/sched/membarrier-sync-core/arch-support.txt +++ b/Documentation/features/sched/membarrier-sync-core/arch-support.txt @@ -10,6 +10,22 @@ # Rely on implicit context synchronization as a result of exception return # when returning from IPI handler, and when returning to user-space. # +# * riscv +# +# riscv uses xRET as return from interrupt and to return to user-space. +# +# Given that xRET is not core serializing, we rely on FENCE.I for providing +# core serialization: +# +# - by calling sync_core_before_usermode() on return from interrupt (cf. +# ipi_sync_core()), +# +# - via switch_mm() and sync_core_before_usermode() (respectively, for +# uthread->uthread and kthread->uthread transitions) before returning +# to user-space. +# +# The serialization in switch_mm() is activated by prepare_sync_core_cmd(). +# # * x86 # # x86-32 uses IRET as return from interrupt, which takes care of the IPI. @@ -43,7 +59,7 @@ | openrisc: | TODO | | parisc: | TODO | | powerpc: | ok | - | riscv: | TODO | + | riscv: | ok | | s390: | ok | | sh: | TODO | | sparc: | TODO | diff --git a/MAINTAINERS b/MAINTAINERS index 78c835cc69c9..cc80968ec355 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -14041,6 +14041,7 @@ L: linux-kernel@vger.kernel.org S: Supported F: Documentation/scheduler/membarrier.rst F: arch/*/include/asm/membarrier.h +F: arch/*/include/asm/sync_core.h F: include/uapi/linux/membarrier.h F: kernel/sched/membarrier.c diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig index 087abf9e51c6..70836381b948 100644 --- a/arch/riscv/Kconfig +++ b/arch/riscv/Kconfig @@ -28,14 +28,17 @@ config RISCV select ARCH_HAS_GIGANTIC_PAGE select ARCH_HAS_KCOV select ARCH_HAS_MEMBARRIER_CALLBACKS + select ARCH_HAS_MEMBARRIER_SYNC_CORE select ARCH_HAS_MMIOWB select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE select ARCH_HAS_PMEM_API + select ARCH_HAS_PREPARE_SYNC_CORE_CMD select ARCH_HAS_PTE_SPECIAL select ARCH_HAS_SET_DIRECT_MAP if MMU 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_SYSCALL_WRAPPER select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST select ARCH_HAS_UBSAN_SANITIZE_ALL diff --git a/arch/riscv/include/asm/membarrier.h b/arch/riscv/include/asm/membarrier.h index 6c016ebb5020..47b240d0d596 100644 --- a/arch/riscv/include/asm/membarrier.h +++ b/arch/riscv/include/asm/membarrier.h @@ -22,6 +22,25 @@ static inline void membarrier_arch_switch_mm(struct mm_struct *prev, /* * The membarrier system call requires a full memory barrier * after storing to rq->curr, before going back to user-space. + * + * This barrier is also needed for the SYNC_CORE command when + * switching between processes; in particular, on a transition + * from a thread belonging to another mm to a thread belonging + * to the mm for which a membarrier SYNC_CORE is done on CPU0: + * + * - [CPU0] sets all bits in the mm icache_stale_mask (in + * prepare_sync_core_cmd()); + * + * - [CPU1] stores to rq->curr (by the scheduler); + * + * - [CPU0] loads rq->curr within membarrier and observes + * cpu_rq(1)->curr->mm != mm, so the IPI is skipped on + * CPU1; this means membarrier relies on switch_mm() to + * issue the sync-core; + * + * - [CPU1] switch_mm() loads icache_stale_mask; if the bit + * is zero, switch_mm() may incorrectly skip the sync-core. + * * Matches a full barrier in the proximity of the membarrier * system call entry. */ diff --git a/arch/riscv/include/asm/sync_core.h b/arch/riscv/include/asm/sync_core.h new file mode 100644 index 000000000000..9153016da8f1 --- /dev/null +++ b/arch/riscv/include/asm/sync_core.h @@ -0,0 +1,29 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _ASM_RISCV_SYNC_CORE_H +#define _ASM_RISCV_SYNC_CORE_H + +/* + * 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"); +} + +#ifdef CONFIG_SMP +/* + * Ensure the next switch_mm() on every CPU issues a core serializing + * instruction for the given @mm. + */ +static inline void prepare_sync_core_cmd(struct mm_struct *mm) +{ + cpumask_setall(&mm->context.icache_stale_mask); +} +#else +static inline void prepare_sync_core_cmd(struct mm_struct *mm) +{ +} +#endif /* CONFIG_SMP */ + +#endif /* _ASM_RISCV_SYNC_CORE_H */ diff --git a/kernel/sched/core.c b/kernel/sched/core.c index a972628e7756..e4a87bcf28d4 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -6721,6 +6721,10 @@ static void __sched notrace __schedule(unsigned int sched_mode) * * The barrier matches a full barrier in the proximity of * the membarrier system call entry. + * + * On RISC-V, this barrier pairing is also needed for the + * SYNC_CORE command when switching between processes, cf. + * the inline comments in membarrier_arch_switch_mm(). */ ++*switch_count; diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c index 6d1f31b3a967..703e8d80a576 100644 --- a/kernel/sched/membarrier.c +++ b/kernel/sched/membarrier.c @@ -342,6 +342,10 @@ static int membarrier_private_expedited(int flags, int cpu_id) /* * Matches memory barriers after rq->curr modification in * scheduler. + * + * On RISC-V, this barrier pairing is also needed for the + * SYNC_CORE command when switching between processes, cf. + * the inline comments in membarrier_arch_switch_mm(). */ smp_mb(); /* system call entry is not a mb. */