PR_RISCV_SET_ICACHE_FLUSH_CTX(2const) PR_RISCV_SET_ICACHE_FLUSH_CTX(2const)

PR_RISCV_SET_ICACHE_FLUSH_CTX - Enable/disable icache flushing instructions in userspace.

Standard C library (libc, -lc)

#include <linux/prctl.h>  /* Definition of PR_* constants */
#include <sys/prctl.h>
int prctl(PR_RISCV_SET_ICACHE_FLUSH_CTX, unsigned long ctx,
          unsigned long scope);

The context and the scope can be provided using ctx and scope respectively.

When scope is set to 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 is set to 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 prctl PR_RISCV_SET_ICACHE_FLUSH_CTX is a nop as migrations across harts will not occur.

The following values for ctx can be specified:

Allow fence.i in user space.
Disallow fence.i in user space. All threads in a process will be affected when scope is set to 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.

The following values for scope can be specified:

Ensure the icache of any thread in this process is coherent with instruction storage upon migration.
Ensure the icache of the current thread is coherent with instruction storage upon migration.

The following files are meant to be compiled and linked with each other. The modify_instruction() function replaces an add with zero with an add with one, causing the instruction sequence in get_value() to change from returning a zero to returning a one.

#include <stdio.h>
#include <sys/prctl.h>
extern int get_value(void);
extern void modify_instruction(void);
int
main(void)
{
    int value = get_value();
    printf("Value before cmodx: %d\n", value);
    // Call prctl before first fence.i is called
    prctl(PR_RISCV_SET_ICACHE_FLUSH_CTX, PR_RISCV_CTX_SW_FENCEI_ON,
          PR_RISCV_SCOPE_PER_PROCESS);
    modify_instruction();
    // Call prctl after final fence.i is called in process
    prctl(PR_RISCV_SET_ICACHE_FLUSH_CTX, PR_RISCV_CTX_SW_FENCEI_OFF,
          PR_RISCV_SCOPE_PER_PROCESS);
    value = get_value();
    printf("Value after cmodx: %d\n", value);
    return 0;
}

.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

Value before cmodx: 0
Value after cmodx: 1

Linux. RISC-V only.

prctl(2)

2024-07-23 Linux man-pages 6.10