[PATCH v2 2/2] documentation: Document PR_RISCV_SET_ICACHE_FLUSH_CTX prctl

[Charlie Jenkins]

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
+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
+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
+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
 

-- 
2.34.1