1.. SPDX-License-Identifier: GPL-2.0 2 3============================================================================== 4Concurrent Modification and Execution of Instructions (CMODX) for RISC-V Linux 5============================================================================== 6 7CMODX is a programming technique where a program executes instructions that were 8modified by the program itself. Instruction storage and the instruction cache 9(icache) are not guaranteed to be synchronized on RISC-V hardware. Therefore, the 10program must enforce its own synchronization with the unprivileged fence.i 11instruction. 12 13However, the default Linux ABI prohibits the use of fence.i in userspace 14applications. At any point the scheduler may migrate a task onto a new hart. If 15migration occurs after the userspace synchronized the icache and instruction 16storage with fence.i, the icache on the new hart will no longer be clean. This 17is due to the behavior of fence.i only affecting the hart that it is called on. 18Thus, the hart that the task has been migrated to may not have synchronized 19instruction storage and icache. 20 21There are two ways to solve this problem: use the riscv_flush_icache() syscall, 22or use the ``PR_RISCV_SET_ICACHE_FLUSH_CTX`` prctl() and emit fence.i in 23userspace. The syscall performs a one-off icache flushing operation. The prctl 24changes the Linux ABI to allow userspace to emit icache flushing operations. 25 26As an aside, "deferred" icache flushes can sometimes be triggered in the kernel. 27At the time of writing, this only occurs during the riscv_flush_icache() syscall 28and when the kernel uses copy_to_user_page(). These deferred flushes happen only 29when the memory map being used by a hart changes. If the prctl() context caused 30an icache flush, this deferred icache flush will be skipped as it is redundant. 31Therefore, there will be no additional flush when using the riscv_flush_icache() 32syscall inside of the prctl() context. 33 34prctl() Interface 35--------------------- 36 37Call prctl() with ``PR_RISCV_SET_ICACHE_FLUSH_CTX`` as the first argument. The 38remaining arguments will be delegated to the riscv_set_icache_flush_ctx 39function detailed below. 40 41.. kernel-doc:: arch/riscv/mm/cacheflush.c 42 :identifiers: riscv_set_icache_flush_ctx 43 44Example usage: 45 46The following files are meant to be compiled and linked with each other. The 47modify_instruction() function replaces an add with 0 with an add with one, 48causing the instruction sequence in get_value() to change from returning a zero 49to returning a one. 50 51cmodx.c:: 52 53 #include <stdio.h> 54 #include <sys/prctl.h> 55 56 extern int get_value(); 57 extern void modify_instruction(); 58 59 int main() 60 { 61 int value = get_value(); 62 printf("Value before cmodx: %d\n", value); 63 64 // Call prctl before first fence.i is called inside modify_instruction 65 prctl(PR_RISCV_SET_ICACHE_FLUSH_CTX, PR_RISCV_CTX_SW_FENCEI_ON, PR_RISCV_SCOPE_PER_PROCESS); 66 modify_instruction(); 67 // Call prctl after final fence.i is called in process 68 prctl(PR_RISCV_SET_ICACHE_FLUSH_CTX, PR_RISCV_CTX_SW_FENCEI_OFF, PR_RISCV_SCOPE_PER_PROCESS); 69 70 value = get_value(); 71 printf("Value after cmodx: %d\n", value); 72 return 0; 73 } 74 75cmodx.S:: 76 77 .option norvc 78 79 .text 80 .global modify_instruction 81 modify_instruction: 82 lw a0, new_insn 83 lui a5,%hi(old_insn) 84 sw a0,%lo(old_insn)(a5) 85 fence.i 86 ret 87 88 .section modifiable, "awx" 89 .global get_value 90 get_value: 91 li a0, 0 92 old_insn: 93 addi a0, a0, 0 94 ret 95 96 .data 97 new_insn: 98 addi a0, a0, 1 99