On Fri, Jan 5, 2024 at 7:52 PM Charlie Jenkins <charlie@xxxxxxxxxxxx> wrote: > > On Fri, Jan 05, 2024 at 06:52:12PM -0800, Atish Patra wrote: > > 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. > > I can highlight the point that the prctl is sticky and the syscall is > not. > > As for "why", they simply fill different roles. It is useful to have > both a sticky and a non-sticky option. I chose the sticky operation to > be a prctl rather than a generic syscall because the semantics of prctl > is that they operate on process or thread scoped behavior which is what > was needed. > Yeah. That's what I was suggesting. It is enough for readers to understand the difference between the two approaches without reading the kernel source code. > - Charlie > > > > > > +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 -- Regards, Atish