[v5,2/2] documentation: Document PR_RISCV_SET_ICACHE_FLUSH_CTX prctl

Commit Message

Charlie Jenkins Jan. 8, 2024, 6:42 p.m. UTC

Provide documentation that explains how to properly do CMODX in riscv.

Signed-off-by: Charlie Jenkins <[e-mail]>
---
 Documentation/arch/riscv/cmodx.rst | 88 ++++++++++++++++++++++++++++++++++++++
 Documentation/arch/riscv/index.rst |  1 +
 2 files changed, 89 insertions(+)

Comments

Atish Patra Jan. 9, 2024, 1:24 a.m. UTC | #1

On Mon, Jan 8, 2024 at 10:42 AM Charlie Jenkins <[e-mail]> wrote:
>
> Provide documentation that explains how to properly do CMODX in riscv.
>
> Signed-off-by: Charlie Jenkins <[e-mail]>
> ---
>  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..afd7086c222c
> --- /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) are not guaranteed to be synchronized on RISC-V hardware. Therefore, the
> +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 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.
> +
> +prctl() Interface
> +---------------------
> +
> +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
> +

Document the arguments of the prctl as well ?

> +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
>
>
> --
> 2.43.0
>


--
Regards,
Atish

Charlie Jenkins Jan. 9, 2024, 2:20 a.m. UTC | #2

On Mon, Jan 08, 2024 at 05:24:47PM -0800, Atish Patra wrote:
> On Mon, Jan 8, 2024 at 10:42 AM Charlie Jenkins <[e-mail]> wrote:
> >
> > Provide documentation that explains how to properly do CMODX in riscv.
> >
> > Signed-off-by: Charlie Jenkins <[e-mail]>
> > ---
> >  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..afd7086c222c
> > --- /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) are not guaranteed to be synchronized on RISC-V hardware. Therefore, the
> > +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 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.
> > +
> > +prctl() Interface
> > +---------------------
> > +
> > +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
> > +
> 
> Document the arguments of the prctl as well ?

Do you mean to include the ``PR_RISCV_SET_ICACHE_FLUSH_CTX`` key in the
comment of riscv_set_icache_flush_ctx? The args to
riscv_set_icache_flush_ctx are the args to the prctl except for the key.

- Charlie

> 
> > +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
> >
> >
> > --
> > 2.43.0
> >
> 
> 
> --
> Regards,
> Atish

Atish Patra Jan. 9, 2024, 7:51 a.m. UTC | #3

On Mon, Jan 8, 2024 at 6:20 PM Charlie Jenkins <[e-mail]> wrote:
>
> On Mon, Jan 08, 2024 at 05:24:47PM -0800, Atish Patra wrote:
> > On Mon, Jan 8, 2024 at 10:42 AM Charlie Jenkins <[e-mail]> wrote:
> > >
> > > Provide documentation that explains how to properly do CMODX in riscv.
> > >
> > > Signed-off-by: Charlie Jenkins <[e-mail]>
> > > ---
> > >  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..afd7086c222c
> > > --- /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) are not guaranteed to be synchronized on RISC-V hardware. Therefore, the
> > > +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 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.
> > > +
> > > +prctl() Interface
> > > +---------------------
> > > +
> > > +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
> > > +
> >
> > Document the arguments of the prctl as well ?
>
> Do you mean to include the ``PR_RISCV_SET_ICACHE_FLUSH_CTX`` key in the
> comment of riscv_set_icache_flush_ctx? The args to
> riscv_set_icache_flush_ctx are the args to the prctl except for the key.
>

No, I mean describe the argument2(ctx) and argument3(per_thread) as well.
Since this is a documentation of the new prctl, we should document all
args so that an user
can use it without grepping through the kernel sources.

> - Charlie
>
> >
> > > +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
> > >
> > >
> > > --
> > > 2.43.0
> > >
> >
> >
> > --
> > Regards,
> > Atish

Atish Patra Jan. 9, 2024, 5:54 p.m. UTC | #4

On Mon, Jan 8, 2024 at 11:51 PM Atish Patra <[e-mail]> wrote:
>
> On Mon, Jan 8, 2024 at 6:20 PM Charlie Jenkins <[e-mail]> wrote:
> >
> > On Mon, Jan 08, 2024 at 05:24:47PM -0800, Atish Patra wrote:
> > > On Mon, Jan 8, 2024 at 10:42 AM Charlie Jenkins <[e-mail]> wrote:
> > > >
> > > > Provide documentation that explains how to properly do CMODX in riscv.
> > > >
> > > > Signed-off-by: Charlie Jenkins <[e-mail]>
> > > > ---
> > > >  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..afd7086c222c
> > > > --- /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) are not guaranteed to be synchronized on RISC-V hardware. Therefore, the
> > > > +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 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.
> > > > +
> > > > +prctl() Interface
> > > > +---------------------
> > > > +
> > > > +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
> > > > +
> > >
> > > Document the arguments of the prctl as well ?
> >
> > Do you mean to include the ``PR_RISCV_SET_ICACHE_FLUSH_CTX`` key in the
> > comment of riscv_set_icache_flush_ctx? The args to
> > riscv_set_icache_flush_ctx are the args to the prctl except for the key.
> >
>
> No, I mean describe the argument2(ctx) and argument3(per_thread) as well.
> Since this is a documentation of the new prctl, we should document all
> args so that an user
> can use it without grepping through the kernel sources.
>

Never mind. I missed the identifiers tag for kernel doc. Thanks for
the reminder.
The patch looks good to me.

Reviewed-by: Atish Patra <[e-mail]>

> > - Charlie
> >
> > >
> > > > +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
> > > >
> > > >
> > > > --
> > > > 2.43.0
> > > >
> > >
> > >
> > > --
> > > Regards,
> > > Atish
>
>
>
> --
> Regards,
> Atish

Patch

diff --git a/Documentation/arch/riscv/cmodx.rst b/Documentation/arch/riscv/cmodx.rst
new file mode 100644
index 000000000000..afd7086c222c
--- /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) are not guaranteed to be synchronized on RISC-V hardware. Therefore, the
+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 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.
+
+prctl() Interface
+---------------------
+
+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