[PATCH v3 2/2] documentation: Document PR_RISCV_SET_ICACHE_FLUSH_CTX prctl

[Atish Patra]

On Wed, Dec 13, 2023 at 2:48 PM Charlie Jenkins <charlie at rivosinc.com> wrote:
>
> Provide documentation that explains how to properly do CMODX in riscv.
>
> Signed-off-by: Charlie Jenkins <charlie at rivosinc.com>
> ---
>  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
>
>
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-- 
Regards,
Atish