Message-ID: <26808f34-d9c4-404a-bf09-45c4aff139ad@ghiti.fr>
Date: Wed, 24 Jan 2024 08:19:42 +0100
From: Alexandre Ghiti <alex@...ti.fr>
To: Charlie Jenkins <charlie@...osinc.com>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Jonathan Corbet <corbet@....net>, Conor Dooley <conor.dooley@...rochip.com>,
 Clément Léger <cleger@...osinc.com>,
 Atish Patra <atishp@...shpatra.org>, Randy Dunlap <rdunlap@...radead.org>
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, Atish Patra <atishp@...osinc.com>
Subject: Re: [PATCH v9 2/2] documentation: Document
 PR_RISCV_SET_ICACHE_FLUSH_CTX prctl

On 24/01/2024 00:29, Charlie Jenkins wrote:
> Provide documentation that explains how to properly do CMODX in riscv.
>
> Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
> Reviewed-by: Atish Patra <atishp@...osinc.com>
> ---
>   Documentation/arch/riscv/cmodx.rst | 96 ++++++++++++++++++++++++++++++++++++++
>   Documentation/arch/riscv/index.rst |  1 +
>   2 files changed, 97 insertions(+)
>
> diff --git a/Documentation/arch/riscv/cmodx.rst b/Documentation/arch/riscv/cmodx.rst
> new file mode 100644
> index 000000000000..2ad46129d812
> --- /dev/null
> +++ b/Documentation/arch/riscv/cmodx.rst
> @@ -0,0 +1,96 @@
> +.. 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


Nit: I think you mean "the icache on the new hart will no longer be clean".


> +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.
> +
> +As an aside, "deferred" icache flushes can sometimes be triggered in the kernel.
> +At the time of writing, this only occurs during the riscv_flush_icache() syscall
> +and when the kernel uses copy_to_user_page(). These deferred flushes happen only
> +when the memory map being used by a hart changes. If the prctl() context caused
> +an icache flush, this deferred icache flush will be skipped as it is redundant.
> +Therefore, there will be no additional flush when using the riscv_flush_icache()
> +syscall inside of the prctl() context.
> +
> +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, PR_RISCV_SCOPE_PER_PROCESS);
> +		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
>   
>

I don't know how man pages are synchronized with new additions in the 
kernel, do you? It would be nice to have this new prctl documented for 
userspace.

Thanks,

Alex

Powered by blists - more mailing lists