diff options
author | Palmer Dabbelt <palmer@rivosinc.com> | 2024-04-17 04:50:58 +0300 |
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committer | Palmer Dabbelt <palmer@rivosinc.com> | 2024-04-30 20:35:42 +0300 |
commit | 4202f62cb64b8a04782435f1006c322af2d2619b (patch) | |
tree | 080886df135391903ff5c530bc08efe5a803dbcc | |
parent | 3f45244289398b6f29074fdb91dd164667bb0960 (diff) | |
parent | decde1fa209323c77a7498e803350c5f3e992d62 (diff) | |
download | linux-4202f62cb64b8a04782435f1006c322af2d2619b.tar.xz |
Merge patch series "riscv: Create and document PR_RISCV_SET_ICACHE_FLUSH_CTX prctl"
Charlie Jenkins <charlie@rivosinc.com> says:
Improve the performance of icache flushing by creating a new prctl flag
PR_RISCV_SET_ICACHE_FLUSH_CTX. The interface is left generic to allow
for future expansions such as with the proposed J extension [1].
Documentation is also provided to explain the use case.
Patch sent to add PR_RISCV_SET_ICACHE_FLUSH_CTX to man-pages [2].
[1] https://github.com/riscv/riscv-j-extension
[2] https://lore.kernel.org/linux-man/20240124-fencei_prctl-v1-1-0bddafcef331@rivosinc.com
* b4-shazam-merge:
cpumask: Add assign cpu
documentation: Document PR_RISCV_SET_ICACHE_FLUSH_CTX prctl
riscv: Include riscv_set_icache_flush_ctx prctl
riscv: Remove unnecessary irqflags processor.h include
Link: https://lore.kernel.org/r/20240312-fencei-v13-0-4b6bdc2bbf32@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
-rw-r--r-- | Documentation/arch/riscv/cmodx.rst | 98 | ||||
-rw-r--r-- | Documentation/arch/riscv/index.rst | 1 | ||||
-rw-r--r-- | arch/riscv/include/asm/irqflags.h | 1 | ||||
-rw-r--r-- | arch/riscv/include/asm/mmu.h | 2 | ||||
-rw-r--r-- | arch/riscv/include/asm/processor.h | 10 | ||||
-rw-r--r-- | arch/riscv/include/asm/switch_to.h | 23 | ||||
-rw-r--r-- | arch/riscv/mm/cacheflush.c | 113 | ||||
-rw-r--r-- | arch/riscv/mm/context.c | 19 | ||||
-rw-r--r-- | include/linux/cpumask.h | 16 | ||||
-rw-r--r-- | include/uapi/linux/prctl.h | 6 | ||||
-rw-r--r-- | kernel/sys.c | 6 |
11 files changed, 286 insertions, 9 deletions
diff --git a/Documentation/arch/riscv/cmodx.rst b/Documentation/arch/riscv/cmodx.rst new file mode 100644 index 000000000000..1c0ca06b6c97 --- /dev/null +++ b/Documentation/arch/riscv/cmodx.rst @@ -0,0 +1,98 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============================================================================== +Concurrent Modification and Execution of Instructions (CMODX) for RISC-V Linux +============================================================================== + +CMODX is a programming technique where a program executes instructions that were +modified by the program itself. Instruction storage and the instruction cache +(icache) are not guaranteed to be synchronized on RISC-V hardware. Therefore, the +program must enforce its own synchronization with the unprivileged fence.i +instruction. + +However, the default Linux ABI prohibits the use of fence.i in userspace +applications. At any point the scheduler may migrate a task onto a new hart. If +migration occurs after the userspace synchronized the icache and instruction +storage with fence.i, the icache on the new hart will no longer be clean. This +is due to the behavior of fence.i only affecting the hart that it is called on. +Thus, the hart that the task has been migrated to may not have synchronized +instruction storage and icache. + +There are two ways to solve this problem: use the riscv_flush_icache() syscall, +or use the ``PR_RISCV_SET_ICACHE_FLUSH_CTX`` prctl() and emit fence.i in +userspace. The syscall performs a one-off icache flushing operation. The prctl +changes the Linux ABI to allow userspace to emit icache flushing operations. + +As an aside, "deferred" icache flushes can sometimes be triggered in the kernel. +At the time of writing, this only occurs during the riscv_flush_icache() syscall +and when the kernel uses copy_to_user_page(). These deferred flushes happen only +when the memory map being used by a hart changes. If the prctl() context caused +an icache flush, this deferred icache flush will be skipped as it is redundant. +Therefore, there will be no additional flush when using the riscv_flush_icache() +syscall inside of the prctl() context. + +prctl() Interface +--------------------- + +Call prctl() with ``PR_RISCV_SET_ICACHE_FLUSH_CTX`` as the first argument. The +remaining arguments will be delegated to the riscv_set_icache_flush_ctx +function detailed below. + +.. kernel-doc:: arch/riscv/mm/cacheflush.c + :identifiers: riscv_set_icache_flush_ctx + +Example usage: + +The following files are meant to be compiled and linked with each other. The +modify_instruction() function replaces an add with 0 with an add with one, +causing the instruction sequence in get_value() to change from returning a zero +to returning a one. + +cmodx.c:: + + #include <stdio.h> + #include <sys/prctl.h> + + extern int get_value(); + extern void modify_instruction(); + + int main() + { + int value = get_value(); + printf("Value before cmodx: %d\n", value); + + // Call prctl before first fence.i is called inside modify_instruction + prctl(PR_RISCV_SET_ICACHE_FLUSH_CTX_ON, PR_RISCV_CTX_SW_FENCEI, PR_RISCV_SCOPE_PER_PROCESS); + modify_instruction(); + // Call prctl after final fence.i is called in process + prctl(PR_RISCV_SET_ICACHE_FLUSH_CTX_OFF, PR_RISCV_CTX_SW_FENCEI, PR_RISCV_SCOPE_PER_PROCESS); + + value = get_value(); + printf("Value after cmodx: %d\n", value); + return 0; + } + +cmodx.S:: + + .option norvc + + .text + .global modify_instruction + modify_instruction: + lw a0, new_insn + lui a5,%hi(old_insn) + sw a0,%lo(old_insn)(a5) + fence.i + ret + + .section modifiable, "awx" + .global get_value + get_value: + li a0, 0 + old_insn: + addi a0, a0, 0 + ret + + .data + new_insn: + addi a0, a0, 1 diff --git a/Documentation/arch/riscv/index.rst b/Documentation/arch/riscv/index.rst index 4dab0cb4b900..eecf347ce849 100644 --- a/Documentation/arch/riscv/index.rst +++ b/Documentation/arch/riscv/index.rst @@ -13,6 +13,7 @@ RISC-V architecture patch-acceptance uabi vector + cmodx features diff --git a/arch/riscv/include/asm/irqflags.h b/arch/riscv/include/asm/irqflags.h index 08d4d6a5b7e9..6fd8cbfcfcc7 100644 --- a/arch/riscv/include/asm/irqflags.h +++ b/arch/riscv/include/asm/irqflags.h @@ -7,7 +7,6 @@ #ifndef _ASM_RISCV_IRQFLAGS_H #define _ASM_RISCV_IRQFLAGS_H -#include <asm/processor.h> #include <asm/csr.h> /* read interrupt enabled status */ diff --git a/arch/riscv/include/asm/mmu.h b/arch/riscv/include/asm/mmu.h index 355504b37f8e..60be458e94da 100644 --- a/arch/riscv/include/asm/mmu.h +++ b/arch/riscv/include/asm/mmu.h @@ -19,6 +19,8 @@ typedef struct { #ifdef CONFIG_SMP /* A local icache flush is needed before user execution can resume. */ cpumask_t icache_stale_mask; + /* Force local icache flush on all migrations. */ + bool force_icache_flush; #endif #ifdef CONFIG_BINFMT_ELF_FDPIC unsigned long exec_fdpic_loadmap; diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h index 0faf5f161f1e..68c3432dc6ea 100644 --- a/arch/riscv/include/asm/processor.h +++ b/arch/riscv/include/asm/processor.h @@ -68,6 +68,7 @@ #endif #ifndef __ASSEMBLY__ +#include <linux/cpumask.h> struct task_struct; struct pt_regs; @@ -122,6 +123,12 @@ struct thread_struct { struct __riscv_v_ext_state vstate; unsigned long align_ctl; struct __riscv_v_ext_state kernel_vstate; +#ifdef CONFIG_SMP + /* Flush the icache on migration */ + bool force_icache_flush; + /* A forced icache flush is not needed if migrating to the previous cpu. */ + unsigned int prev_cpu; +#endif }; /* Whitelist the fstate from the task_struct for hardened usercopy */ @@ -183,6 +190,9 @@ extern int set_unalign_ctl(struct task_struct *tsk, unsigned int val); #define GET_UNALIGN_CTL(tsk, addr) get_unalign_ctl((tsk), (addr)) #define SET_UNALIGN_CTL(tsk, val) set_unalign_ctl((tsk), (val)) +#define RISCV_SET_ICACHE_FLUSH_CTX(arg1, arg2) riscv_set_icache_flush_ctx(arg1, arg2) +extern int riscv_set_icache_flush_ctx(unsigned long ctx, unsigned long per_thread); + #endif /* __ASSEMBLY__ */ #endif /* _ASM_RISCV_PROCESSOR_H */ diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h index 7efdb0584d47..7594df37cc9f 100644 --- a/arch/riscv/include/asm/switch_to.h +++ b/arch/riscv/include/asm/switch_to.h @@ -8,6 +8,7 @@ #include <linux/jump_label.h> #include <linux/sched/task_stack.h> +#include <linux/mm_types.h> #include <asm/vector.h> #include <asm/cpufeature.h> #include <asm/processor.h> @@ -72,14 +73,36 @@ static __always_inline bool has_fpu(void) { return false; } extern struct task_struct *__switch_to(struct task_struct *, struct task_struct *); +static inline bool switch_to_should_flush_icache(struct task_struct *task) +{ +#ifdef CONFIG_SMP + bool stale_mm = task->mm && task->mm->context.force_icache_flush; + bool stale_thread = task->thread.force_icache_flush; + bool thread_migrated = smp_processor_id() != task->thread.prev_cpu; + + return thread_migrated && (stale_mm || stale_thread); +#else + return false; +#endif +} + +#ifdef CONFIG_SMP +#define __set_prev_cpu(thread) ((thread).prev_cpu = smp_processor_id()) +#else +#define __set_prev_cpu(thread) +#endif + #define switch_to(prev, next, last) \ do { \ struct task_struct *__prev = (prev); \ struct task_struct *__next = (next); \ + __set_prev_cpu(__prev->thread); \ if (has_fpu()) \ __switch_to_fpu(__prev, __next); \ if (has_vector()) \ __switch_to_vector(__prev, __next); \ + if (switch_to_should_flush_icache(__next)) \ + local_flush_icache_all(); \ ((last) = __switch_to(__prev, __next)); \ } while (0) diff --git a/arch/riscv/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c index bc61ee5975e4..3ef666c7dfc7 100644 --- a/arch/riscv/mm/cacheflush.c +++ b/arch/riscv/mm/cacheflush.c @@ -5,6 +5,7 @@ #include <linux/acpi.h> #include <linux/of.h> +#include <linux/prctl.h> #include <asm/acpi.h> #include <asm/cacheflush.h> @@ -152,3 +153,115 @@ void __init riscv_init_cbo_blocksizes(void) if (cboz_block_size) riscv_cboz_block_size = cboz_block_size; } + +#ifdef CONFIG_SMP +static void set_icache_stale_mask(void) +{ + cpumask_t *mask; + bool stale_cpu; + + /* + * Mark every other hart's icache as needing a flush for + * this MM. Maintain the previous value of the current + * cpu to handle the case when this function is called + * concurrently on different harts. + */ + mask = ¤t->mm->context.icache_stale_mask; + stale_cpu = cpumask_test_cpu(smp_processor_id(), mask); + + cpumask_setall(mask); + cpumask_assign_cpu(smp_processor_id(), mask, stale_cpu); +} +#endif + +/** + * riscv_set_icache_flush_ctx() - Enable/disable icache flushing instructions in + * userspace. + * @ctx: Set the type of icache flushing instructions permitted/prohibited in + * userspace. Supported values described below. + * + * Supported values for ctx: + * + * * %PR_RISCV_CTX_SW_FENCEI_ON: Allow fence.i in user space. + * + * * %PR_RISCV_CTX_SW_FENCEI_OFF: Disallow fence.i in user space. All threads in + * a process will be affected when ``scope == PR_RISCV_SCOPE_PER_PROCESS``. + * Therefore, caution must be taken; use this flag only when you can guarantee + * that no thread in the process will emit fence.i from this point onward. + * + * @scope: Set scope of where icache flushing instructions are allowed to be + * emitted. Supported values described below. + * + * Supported values for scope: + * + * * %PR_RISCV_SCOPE_PER_PROCESS: Ensure the icache of any thread in this process + * is coherent with instruction storage upon + * migration. + * + * * %PR_RISCV_SCOPE_PER_THREAD: Ensure the icache of the current thread is + * coherent with instruction storage upon + * migration. + * + * When ``scope == PR_RISCV_SCOPE_PER_PROCESS``, all threads in the process are + * permitted to emit icache flushing instructions. Whenever any thread in the + * process is migrated, the corresponding hart's icache will be guaranteed to be + * consistent with instruction storage. This does not enforce any guarantees + * outside of migration. If a thread modifies an instruction that another thread + * may attempt to execute, the other thread must still emit an icache flushing + * instruction before attempting to execute the potentially modified + * instruction. This must be performed by the user-space program. + * + * In per-thread context (eg. ``scope == PR_RISCV_SCOPE_PER_THREAD``) only the + * thread calling this function is permitted to emit icache flushing + * instructions. When the thread is migrated, the corresponding hart's icache + * will be guaranteed to be consistent with instruction storage. + * + * On kernels configured without SMP, this function is a nop as migrations + * across harts will not occur. + */ +int riscv_set_icache_flush_ctx(unsigned long ctx, unsigned long scope) +{ +#ifdef CONFIG_SMP + switch (ctx) { + case PR_RISCV_CTX_SW_FENCEI_ON: + switch (scope) { + case PR_RISCV_SCOPE_PER_PROCESS: + current->mm->context.force_icache_flush = true; + break; + case PR_RISCV_SCOPE_PER_THREAD: + current->thread.force_icache_flush = true; + break; + default: + return -EINVAL; + } + break; + case PR_RISCV_CTX_SW_FENCEI_OFF: + switch (scope) { + case PR_RISCV_SCOPE_PER_PROCESS: + current->mm->context.force_icache_flush = false; + + set_icache_stale_mask(); + break; + case PR_RISCV_SCOPE_PER_THREAD: + current->thread.force_icache_flush = false; + + set_icache_stale_mask(); + break; + default: + return -EINVAL; + } + break; + default: + return -EINVAL; + } + return 0; +#else + switch (ctx) { + case PR_RISCV_CTX_SW_FENCEI_ON: + case PR_RISCV_CTX_SW_FENCEI_OFF: + return 0; + default: + return -EINVAL; + } +#endif +} diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c index ba8eb3944687..9b6db024103b 100644 --- a/arch/riscv/mm/context.c +++ b/arch/riscv/mm/context.c @@ -15,6 +15,7 @@ #include <asm/tlbflush.h> #include <asm/cacheflush.h> #include <asm/mmu_context.h> +#include <asm/switch_to.h> #ifdef CONFIG_MMU @@ -297,21 +298,23 @@ static inline void set_mm(struct mm_struct *prev, * * The "cpu" argument must be the current local CPU number. */ -static inline void flush_icache_deferred(struct mm_struct *mm, unsigned int cpu) +static inline void flush_icache_deferred(struct mm_struct *mm, unsigned int cpu, + struct task_struct *task) { #ifdef CONFIG_SMP - cpumask_t *mask = &mm->context.icache_stale_mask; - - if (cpumask_test_cpu(cpu, mask)) { - cpumask_clear_cpu(cpu, mask); + if (cpumask_test_and_clear_cpu(cpu, &mm->context.icache_stale_mask)) { /* * Ensure the remote hart's writes are visible to this hart. * This pairs with a barrier in flush_icache_mm. */ smp_mb(); - local_flush_icache_all(); - } + /* + * If cache will be flushed in switch_to, no need to flush here. + */ + if (!(task && switch_to_should_flush_icache(task))) + local_flush_icache_all(); + } #endif } @@ -334,5 +337,5 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next, set_mm(prev, next, cpu); - flush_icache_deferred(next, cpu); + flush_icache_deferred(next, cpu, task); } diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h index 1c29947db848..9a345d030e4f 100644 --- a/include/linux/cpumask.h +++ b/include/linux/cpumask.h @@ -494,6 +494,22 @@ static __always_inline void __cpumask_clear_cpu(int cpu, struct cpumask *dstp) } /** + * cpumask_assign_cpu - assign a cpu in a cpumask + * @cpu: cpu number (< nr_cpu_ids) + * @dstp: the cpumask pointer + * @bool: the value to assign + */ +static __always_inline void cpumask_assign_cpu(int cpu, struct cpumask *dstp, bool value) +{ + assign_bit(cpumask_check(cpu), cpumask_bits(dstp), value); +} + +static __always_inline void __cpumask_assign_cpu(int cpu, struct cpumask *dstp, bool value) +{ + __assign_bit(cpumask_check(cpu), cpumask_bits(dstp), value); +} + +/** * cpumask_test_cpu - test for a cpu in a cpumask * @cpu: cpu number (< nr_cpu_ids) * @cpumask: the cpumask pointer diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h index 370ed14b1ae0..524d546d697b 100644 --- a/include/uapi/linux/prctl.h +++ b/include/uapi/linux/prctl.h @@ -306,4 +306,10 @@ struct prctl_mm_map { # define PR_RISCV_V_VSTATE_CTRL_NEXT_MASK 0xc # define PR_RISCV_V_VSTATE_CTRL_MASK 0x1f +#define PR_RISCV_SET_ICACHE_FLUSH_CTX 71 +# define PR_RISCV_CTX_SW_FENCEI_ON 0 +# define PR_RISCV_CTX_SW_FENCEI_OFF 1 +# define PR_RISCV_SCOPE_PER_PROCESS 0 +# define PR_RISCV_SCOPE_PER_THREAD 1 + #endif /* _LINUX_PRCTL_H */ diff --git a/kernel/sys.c b/kernel/sys.c index f8e543f1e38a..1b7bda0722ca 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -146,6 +146,9 @@ #ifndef RISCV_V_GET_CONTROL # define RISCV_V_GET_CONTROL() (-EINVAL) #endif +#ifndef RISCV_SET_ICACHE_FLUSH_CTX +# define RISCV_SET_ICACHE_FLUSH_CTX(a, b) (-EINVAL) +#endif /* * this is where the system-wide overflow UID and GID are defined, for @@ -2757,6 +2760,9 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, case PR_RISCV_V_GET_CONTROL: error = RISCV_V_GET_CONTROL(); break; + case PR_RISCV_SET_ICACHE_FLUSH_CTX: + error = RISCV_SET_ICACHE_FLUSH_CTX(arg2, arg3); + break; default: error = -EINVAL; break; |