On Sun, Jan 07, 2024 at 11:06:34PM -0800, Randy Dunlap wrote:
> 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?
Not needed, thank you.
- Charlie
>
> > +
> > +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
