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

[Charlie Jenkins]

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 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.

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.

- 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
> >
> >
> > _______________________________________________
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> > linux-riscv at lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-riscv
> 
> 
> 
> -- 
> Regards,
> Atish