On Wed, Dec 13, 2023 at 2:48 PM Charlie Jenkins <charlie@xxxxxxxxxxxx> wrote: > > 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/ /s/synchonization/synchronization > +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 The syscall is a one time operation while prctl is sticky. It would be great if we can add a little more context why the syscall behaves this way compared to prctl. > +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 /s/synchonizing/synchronizing > +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 > > > _______________________________________________ > linux-riscv mailing list > linux-riscv@xxxxxxxxxxxxxxxxxxx > http://lists.infradead.org/mailman/listinfo/linux-riscv -- Regards, Atish