[PATCH 0/2] riscv: Create and document PR_RISCV_SET_ICACHE_FLUSH_CTX prctl

[PATCH 0/2] riscv: Create and document PR_RISCV_SET_ICACHE_FLUSH_CTX prctl

[Charlie Jenkins]

Improve the performance of icache flushing by creating a new prctl flag
PR_RISCV_SET_ICACHE_FLUSH_CTX. The interface is left generic to allow
for future expansions such as with the proposed J extension [1].

Documentation is also provided to explain the use case.

[1] https://github.com/riscv/riscv-j-extension

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
---
Charlie Jenkins (2):
      riscv: Include riscv_set_icache_flush_ctx prctl
      documentation: Document PR_RISCV_SET_ICACHE_FLUSH_CTX prctl

 Documentation/arch/riscv/cmodx.rst | 98 ++++++++++++++++++++++++++++++++++++++
 Documentation/arch/riscv/index.rst |  1 +
 arch/riscv/include/asm/mmu.h       |  2 +
 arch/riscv/include/asm/processor.h |  6 +++
 arch/riscv/mm/cacheflush.c         | 37 ++++++++++++++
 arch/riscv/mm/context.c            |  8 ++--
 include/uapi/linux/prctl.h         |  3 ++
 kernel/sys.c                       |  6 +++
 8 files changed, 158 insertions(+), 3 deletions(-)
---
base-commit: b85ea95d086471afb4ad062012a4d73cd328fa86
change-id: 20231117-fencei-f9f60d784fa0
-- 
- Charlie

[PATCH 1/2] riscv: Include riscv_set_icache_flush_ctx prctl

[Charlie Jenkins]

Support new prctl with key PR_RISCV_SET_ICACHE_FLUSH_CTX to enable
optimization of cross modifying code. This prctl enables userspace code
to use icache flushing instructions such as fence.i with the guarantee
that the icache will continue to be clean after thread migration.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
---
 arch/riscv/include/asm/mmu.h       |  2 ++
 arch/riscv/include/asm/processor.h |  6 ++++++
 arch/riscv/mm/cacheflush.c         | 37 +++++++++++++++++++++++++++++++++++++
 arch/riscv/mm/context.c            |  8 +++++---
 include/uapi/linux/prctl.h         |  3 +++
 kernel/sys.c                       |  6 ++++++
 6 files changed, 59 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/include/asm/mmu.h b/arch/riscv/include/asm/mmu.h
index 355504b37f8e..60be458e94da 100644
--- a/arch/riscv/include/asm/mmu.h
+++ b/arch/riscv/include/asm/mmu.h
@@ -19,6 +19,8 @@ typedef struct {
 #ifdef CONFIG_SMP
 	/* A local icache flush is needed before user execution can resume. */
 	cpumask_t icache_stale_mask;
+	/* Force local icache flush on all migrations. */
+	bool force_icache_flush;
 #endif
 #ifdef CONFIG_BINFMT_ELF_FDPIC
 	unsigned long exec_fdpic_loadmap;
diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index f19f861cda54..7eda6c75e0f2 100644
--- a/arch/riscv/include/asm/processor.h
+++ b/arch/riscv/include/asm/processor.h
@@ -84,6 +84,9 @@ struct thread_struct {
 	unsigned long vstate_ctrl;
 	struct __riscv_v_ext_state vstate;
 	unsigned long align_ctl;
+#ifdef CONFIG_SMP
+	bool force_icache_flush;
+#endif
 };
 
 /* Whitelist the fstate from the task_struct for hardened usercopy */
@@ -145,6 +148,9 @@ extern int set_unalign_ctl(struct task_struct *tsk, unsigned int val);
 #define GET_UNALIGN_CTL(tsk, addr)	get_unalign_ctl((tsk), (addr))
 #define SET_UNALIGN_CTL(tsk, val)	set_unalign_ctl((tsk), (val))
 
+#define RISCV_SET_ICACHE_FLUSH_CTX(arg1, arg2)	riscv_set_icache_flush_ctx(arg1, arg2)
+extern int riscv_set_icache_flush_ctx(unsigned long ctx, unsigned long per_thread);
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* _ASM_RISCV_PROCESSOR_H */
diff --git a/arch/riscv/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c
index 55a34f2020a8..36f69c71026a 100644
--- a/arch/riscv/mm/cacheflush.c
+++ b/arch/riscv/mm/cacheflush.c
@@ -5,6 +5,7 @@
 
 #include <linux/acpi.h>
 #include <linux/of.h>
+#include <linux/prctl.h>
 #include <asm/acpi.h>
 #include <asm/cacheflush.h>
 
@@ -152,3 +153,39 @@ void __init riscv_init_cbo_blocksizes(void)
 	if (cboz_block_size)
 		riscv_cboz_block_size = cboz_block_size;
 }
+
+/**
+ * Enable userspace to emit icache flushing instructions.
+ *
+ * When in per-process context, there may be multiple threads using the same mm.
+ * Therefore, the icache can never be assumed clean when. Multiple threads in
+ * the process may modify instructions in the mm concurrently.
+ *
+ * In per-thread context, it can be assumed that all modifications to
+ * instructions in memory will be performed by this thread. When the thread is
+ * migrated the icache will be flushed.
+ *
+ * @arg arg: Sets the type of context
+ *  - PR_RISCV_CTX_SW_FENCEI: Allow fence.i in userspace. Another fence.i will
+ *			      emitted on thread/process migration.
+ * @arg per_thread: When set to 0, will use the default behavior of setting the
+ *  icache flush context per process. When set to 1, will use a per thread
+ *  context.
+ */
+int riscv_set_icache_flush_ctx(unsigned long ctx, unsigned long per_thread)
+{
+#ifdef CONFIG_SMP
+	switch (ctx) {
+	case PR_RISCV_CTX_SW_FENCEI:
+		if (per_thread)
+			current->thread.force_icache_flush = true;
+		else
+			current->mm->context.force_icache_flush = true;
+		break;
+
+	default:
+		break;
+	}
+#endif
+	return 0;
+}
diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
index 217fd4de6134..a394b146e78a 100644
--- a/arch/riscv/mm/context.c
+++ b/arch/riscv/mm/context.c
@@ -297,12 +297,14 @@ static inline void set_mm(struct mm_struct *prev,
  *
  * The "cpu" argument must be the current local CPU number.
  */
-static inline void flush_icache_deferred(struct mm_struct *mm, unsigned int cpu)
+static inline void flush_icache_deferred(struct mm_struct *mm, unsigned int cpu,
+					 struct task_struct *task)
 {
 #ifdef CONFIG_SMP
 	cpumask_t *mask = &mm->context.icache_stale_mask;
 
-	if (cpumask_test_cpu(cpu, mask)) {
+	if (cpumask_test_cpu(cpu, mask) || mm->context.force_icache_flush ||
+	    mm->context.force_icache_flush) {
 		cpumask_clear_cpu(cpu, mask);
 		/*
 		 * Ensure the remote hart's writes are visible to this hart.
@@ -332,5 +334,5 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 
 	set_mm(prev, next, cpu);
 
-	flush_icache_deferred(next, cpu);
+	flush_icache_deferred(next, cpu, task);
 }
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 370ed14b1ae0..472801ea78cc 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -306,4 +306,7 @@ struct prctl_mm_map {
 # define PR_RISCV_V_VSTATE_CTRL_NEXT_MASK	0xc
 # define PR_RISCV_V_VSTATE_CTRL_MASK		0x1f
 
+#define PR_RISCV_SET_ICACHE_FLUSH_CTX	71
+# define PR_RISCV_CTX_SW_FENCEI		0
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/sys.c b/kernel/sys.c
index 420d9cb9cc8e..e806a8a67c36 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -146,6 +146,9 @@
 #ifndef RISCV_V_GET_CONTROL
 # define RISCV_V_GET_CONTROL()		(-EINVAL)
 #endif
+#ifndef RISCV_SET_ICACHE_FLUSH_CTX
+# define RISCV_SET_ICACHE_FLUSH_CTX(a, b)	(-EINVAL)
+#endif
 
 /*
  * this is where the system-wide overflow UID and GID are defined, for
@@ -2739,6 +2742,9 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 	case PR_RISCV_V_GET_CONTROL:
 		error = RISCV_V_GET_CONTROL();
 		break;
+	case PR_RISCV_SET_ICACHE_FLUSH_CTX:
+		error = RISCV_SET_ICACHE_FLUSH_CTX(arg2, arg3);
+		break;
 	default:
 		error = -EINVAL;
 		break;

-- 
2.42.0

Re: [PATCH 1/2] riscv: Include riscv_set_icache_flush_ctx prctl

[Conor Dooley]

[-- Attachment #1: Type: text/plain, Size: 1256 bytes --]

On Wed, Nov 22, 2023 at 05:03:20PM -0800, Charlie Jenkins wrote:
> +
> +/**
> + * Enable userspace to emit icache flushing instructions.
> + *
> + * When in per-process context, there may be multiple threads using the same mm.
> + * Therefore, the icache can never be assumed clean when. Multiple threads in
> + * the process may modify instructions in the mm concurrently.
> + *
> + * In per-thread context, it can be assumed that all modifications to
> + * instructions in memory will be performed by this thread. When the thread is
> + * migrated the icache will be flushed.
> + *
> + * @arg arg: Sets the type of context
> + *  - PR_RISCV_CTX_SW_FENCEI: Allow fence.i in userspace. Another fence.i will
> + *			      emitted on thread/process migration.
> + * @arg per_thread: When set to 0, will use the default behavior of setting the
> + *  icache flush context per process. When set to 1, will use a per thread
> + *  context.
> + */

FYI, this is not valid kerneldoc and breaks allmodconfig builds:
 arch/riscv/mm/cacheflush.c:159: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst

> +int riscv_set_icache_flush_ctx(unsigned long ctx, unsigned long per_thread)


Cheers,
Conor.

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 228 bytes --]

Re: [PATCH 1/2] riscv: Include riscv_set_icache_flush_ctx prctl

[Charlie Jenkins]

On Mon, Nov 27, 2023 at 10:44:54AM +0000, Conor Dooley wrote:
> On Wed, Nov 22, 2023 at 05:03:20PM -0800, Charlie Jenkins wrote:
> > +
> > +/**
> > + * Enable userspace to emit icache flushing instructions.
> > + *
> > + * When in per-process context, there may be multiple threads using the same mm.
> > + * Therefore, the icache can never be assumed clean when. Multiple threads in
> > + * the process may modify instructions in the mm concurrently.
> > + *
> > + * In per-thread context, it can be assumed that all modifications to
> > + * instructions in memory will be performed by this thread. When the thread is
> > + * migrated the icache will be flushed.
> > + *
> > + * @arg arg: Sets the type of context
> > + *  - PR_RISCV_CTX_SW_FENCEI: Allow fence.i in userspace. Another fence.i will
> > + *			      emitted on thread/process migration.
> > + * @arg per_thread: When set to 0, will use the default behavior of setting the
> > + *  icache flush context per process. When set to 1, will use a per thread
> > + *  context.
> > + */
> 
> FYI, this is not valid kerneldoc and breaks allmodconfig builds:
>  arch/riscv/mm/cacheflush.c:159: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
> 
> > +int riscv_set_icache_flush_ctx(unsigned long ctx, unsigned long per_thread)
> 
> 
> Cheers,
> Conor.

Thank you, I will fix that in the next version.

- Charlie

[PATCH 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@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
 

-- 
2.42.0