[PATCH v9 2/2] documentation: Document PR_RISCV_SET_ICACHE_FLUSH_CTX prctl

[Charlie Jenkins]

On Wed, Jan 24, 2024 at 08:19:42AM +0100, Alexandre Ghiti wrote:
> 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 at rivosinc.com>
> > Reviewed-by: Atish Patra <atishp at rivosinc.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".

Aw yes, that should be more explicit.

- Charlie

> 
> 
> > +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
>