Provide documentation that explains how to properly do CMODX in riscv. Signed-off-by: Charlie Jenkins <charlie@xxxxxxxxxxxx> --- Documentation/arch/riscv/cmodx.rst | 98 ++++++++++++++++++++++++++++++++++++++ Documentation/arch/riscv/index.rst | 1 + 2 files changed, 99 insertions(+) diff --git a/Documentation/arch/riscv/cmodx.rst b/Documentation/arch/riscv/cmodx.rst new file mode 100644 index 000000000000..20f327d85116 --- /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) is not guaranteed to be synchronized on RISC-V hardware. Therefore, the +program must enforce its own synchonization 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 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(). The syscall should be used +when the application very rarely needs to flush the icache. If the icache will +need to be flushed many times in the lifetime of the application, the prctl +should be used. + +The prctl informs the kernel that it must emit synchronizing instructions upon +task migration. The program itself must emit synchonizing instructions when +necessary as well. + +1. prctl() Interface +--------------------- + +Before the program emits their first icache flushing instruction, the program +must call this prctl(). + +* prctl(PR_RISCV_SET_ICACHE_FLUSH_CTX, unsigned long ctx, unsigned long per_thread) + + Sets the icache flushing context. If per_thread is 0, context will be + applied per process, otherwise if per_thread is 1 context will be + per-thread. Any other number will have undefined behavior. + + * :c:macro:`PR_RISCV_CTX_SW_FENCEI`: Allow fence.i to be called in + userspace. + +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, PR_RISCV_CTX_SW_FENCEI, 0); + modify_instruction(); + + 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 -- 2.43.0