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From: Atish Patra <atishp@atishpatra.org>
Date: Mon, 8 Jan 2024 23:51:54 -0800
Message-ID: <CAOnJCUL60H16edo6icR-1bzTVk0Tdr+hRf3izharUEEvxCrwhA@mail.gmail.com>
Subject: Re: [PATCH v5 2/2] documentation: Document PR_RISCV_SET_ICACHE_FLUSH_CTX
 prctl
To: Charlie Jenkins <charlie@rivosinc.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>, Palmer Dabbelt <palmer@dabbelt.com>, 
	Albert Ou <aou@eecs.berkeley.edu>, Jonathan Corbet <corbet@lwn.net>, 
	Conor Dooley <conor.dooley@microchip.com>, =?UTF-8?B?Q2zDqW1lbnQgTMOpZ2Vy?= <cleger@rivosinc.com>, 
	Randy Dunlap <rdunlap@infradead.org>, linux-riscv@lists.infradead.org, 
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org
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On Mon, Jan 8, 2024 at 6:20=E2=80=AFPM Charlie Jenkins <charlie@rivosinc.co=
m> wrote:
>
> On Mon, Jan 08, 2024 at 05:24:47PM -0800, Atish Patra wrote:
> > On Mon, Jan 8, 2024 at 10:42=E2=80=AFAM Charlie Jenkins <charlie@rivosi=
nc.com> wrote:
> > >
> > > Provide documentation that explains how to properly do CMODX in riscv=
.
> > >
> > > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> > > ---
> > >  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
> > > +
> > > +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D
> > > +Concurrent Modification and Execution of Instructions (CMODX) for RI=
SC-V Linux
> > > +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D
> > > +
> > > +CMODX is a programming technique where a program executes instructio=
ns that were
> > > +modified by the program itself. Instruction storage and the instruct=
ion cache
> > > +(icache) are not guaranteed to be synchronized on RISC-V hardware. T=
herefore, the
> > > +program must enforce its own synchronization with the unprivileged f=
ence.i
> > > +instruction.
> > > +
> > > +However, the default Linux ABI prohibits the use of fence.i in users=
pace
> > > +applications. At any point the scheduler may migrate a task onto a n=
ew hart. If
> > > +migration occurs after the userspace synchronized the icache and ins=
truction
> > > +storage with fence.i, the icache will no longer be clean. This is du=
e to the
> > > +behavior of fence.i only affecting the hart that it is called on. Th=
us, the hart
> > > +that the task has been migrated to may not have synchronized instruc=
tion 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 ope=
rations.
> > > +
> > > +prctl() Interface
> > > +---------------------
> > > +
> > > +Call prctl() with ``PR_RISCV_SET_ICACHE_FLUSH_CTX`` as the first arg=
ument. 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 ot=
her. The
> > > +modify_instruction() function replaces an add with 0 with an add wit=
h one,
> > > +causing the instruction sequence in get_value() to change from retur=
ning 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 =3D get_value();
> > > +               printf("Value before cmodx: %d\n", value);
> > > +
> > > +               // Call prctl before first fence.i is called inside m=
odify_instruction
> > > +               prctl(PR_RISCV_SET_ICACHE_FLUSH_CTX_ON, PR_RISCV_CTX_=
SW_FENCEI, 0);
> > > +               modify_instruction();
> > > +
> > > +               value =3D 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


--=20
Regards,
Atish