1 .. SPDX-License-Identifier: GPL-2.0
2
3 ==============================================
4 Concurrent Modification and Execution of Instr
5 ==============================================
6
7 CMODX is a programming technique where a progr
8 modified by the program itself. Instruction st
9 (icache) are not guaranteed to be synchronized
10 program must enforce its own synchronization w
11 instruction.
12
13 However, the default Linux ABI prohibits the u
14 applications. At any point the scheduler may m
15 migration occurs after the userspace synchroni
16 storage with fence.i, the icache on the new ha
17 is due to the behavior of fence.i only affecti
18 Thus, the hart that the task has been migrated
19 instruction storage and icache.
20
21 There are two ways to solve this problem: use
22 or use the ``PR_RISCV_SET_ICACHE_FLUSH_CTX`` p
23 userspace. The syscall performs a one-off icac
24 changes the Linux ABI to allow userspace to em
25
26 As an aside, "deferred" icache flushes can som
27 At the time of writing, this only occurs durin
28 and when the kernel uses copy_to_user_page().
29 when the memory map being used by a hart chang
30 an icache flush, this deferred icache flush wi
31 Therefore, there will be no additional flush w
32 syscall inside of the prctl() context.
33
34 prctl() Interface
35 ---------------------
36
37 Call prctl() with ``PR_RISCV_SET_ICACHE_FLUSH_
38 remaining arguments will be delegated to the r
39 function detailed below.
40
41 .. kernel-doc:: arch/riscv/mm/cacheflush.c
42 :identifiers: riscv_set_icache_flush_c
43
44 Example usage:
45
46 The following files are meant to be compiled a
47 modify_instruction() function replaces an add
48 causing the instruction sequence in get_value(
49 to returning a one.
50
51 cmodx.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
63
64 // Call prctl before first fen
65 prctl(PR_RISCV_SET_ICACHE_FLUS
66 modify_instruction();
67 // Call prctl after final fenc
68 prctl(PR_RISCV_SET_ICACHE_FLUS
69
70 value = get_value();
71 printf("Value after cmodx: %d\
72 return 0;
73 }
74
75 cmodx.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
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