Hi-- On 1/7/24 22:21, Charlie Jenkins wrote: > Provide documentation that explains how to properly do CMODX in riscv. > > Signed-off-by: Charlie Jenkins <charlie@xxxxxxxxxxxx> > --- > Documentation/arch/riscv/cmodx.rst | 88 ++++++++++++++++++++++++++++++++++++++ > Documentation/arch/riscv/index.rst | 1 + > 2 files changed, 89 insertions(+) > > diff --git a/Documentation/arch/riscv/cmodx.rst b/Documentation/arch/riscv/cmodx.rst > new file mode 100644 > index 000000000000..71598850e131 > --- /dev/null > +++ b/Documentation/arch/riscv/cmodx.rst > @@ -0,0 +1,88 @@ > +.. 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 are not > +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 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 to may not > +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. > + > +1. prctl() Interface > +--------------------- Why is "1." needed here? or is it? > + > +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, 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 > > Thanks. -- #Randy