Merge patch series "riscv: ftrace: atmoic patching and preempt improvements"

Andy Chiu <andybnac@gmail.com> says:

This series makes atomic code patching in ftrace possible and eliminates
the need of the stop_machine dance. The major difference of this version
is that we merge the CALL_OPS support from Puranjay [1] and make direct
calls available for practical uses such as BPF. Thanks for the time
reviewing the series and suggestions, we hope this version gets a step
closer to happening in the upstream.

Please reference the link to v3 below for more introductory view of the
implementation [2]

Added patch: 2, 4, 10, 11, 12
Modified patch: 5, 6
Unchanged patch: 1, 3, 7, 8, 9
(1, 8 has commit msg modified)

Special thanks to Björn for his efforts on testing and guiding the
series!

[1]: https://lore.kernel.org/lkml/20240306165904.108141-1-puranjay12@gmail.com/
[2]: https://lore.kernel.org/linux-riscv/20241127172908.17149-1-andybnac@gmail.com/

* patches from https://lore.kernel.org/r/20250407180838.42877-1-andybnac@gmail.com:
  riscv: Documentation: add a description about dynamic ftrace
  riscv: ftrace: support direct call using call_ops
  riscv: Implement HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS
  riscv: ftrace: support PREEMPT
  riscv: add a data fence for CMODX in the kernel mode
  riscv: vector: Support calling schedule() for preemptible Vector
  riscv: ftrace: do not use stop_machine to update code
  riscv: ftrace: prepare ftrace for atomic code patching
  kernel: ftrace: export ftrace_sync_ipi
  riscv: ftrace: align patchable functions to 4 Byte boundary
  riscv: ftrace factor out code defined by !WITH_ARG
  riscv: ftrace: support fastcc in Clang for WITH_ARGS

Link: https://lore.kernel.org/r/20250407180838.42877-1-andybnac@gmail.com
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>

Author: Alexandre Ghiti

Documentation/arch/riscv/cmodx.rst

@@ -10,13 +10,45 @@ modified by the program itself. Instruction storage and the instruction cache
 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 on the new hart 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.
+CMODX in the Kernel Space
+---------------------
+
+Dynamic ftrace
+---------------------
+
+Essentially, dynamic ftrace directs the control flow by inserting a function
+call at each patchable function entry, and patches it dynamically at runtime to
+enable or disable the redirection. In the case of RISC-V, 2 instructions,
+AUIPC + JALR, are required to compose a function call. However, it is impossible
+to patch 2 instructions and expect that a concurrent read-side executes them
+without a race condition. This series makes atmoic code patching possible in
+RISC-V ftrace. Kernel preemption makes things even worse as it allows the old
+state to persist across the patching process with stop_machine().
+
+In order to get rid of stop_machine() and run dynamic ftrace with full kernel
+preemption, we partially initialize each patchable function entry at boot-time,
+setting the first instruction to AUIPC, and the second to NOP. Now, atmoic
+patching is possible because the kernel only has to update one instruction.
+According to Ziccif, as long as an instruction is naturally aligned, the ISA
+guarantee an  atomic update.
+
+By fixing down the first instruction, AUIPC, the range of the ftrace trampoline
+is limited to +-2K from the predetermined target, ftrace_caller, due to the lack
+of immediate encoding space in RISC-V. To address the issue, we introduce
+CALL_OPS, where an 8B naturally align metadata is added in front of each
+pacthable function. The metadata is resolved at the first trampoline, then the
+execution can be derect to another custom trampoline.
+
+CMODX in the User Space
+---------------------
+
+Though fence.i is an unprivileged instruction, 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 on the
+new hart 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() and emit fence.i in

arch/riscv/Kconfig

@@ -99,6 +99,7 @@ config RISCV
 	select EDAC_SUPPORT
 	select FRAME_POINTER if PERF_EVENTS || (FUNCTION_TRACER && !DYNAMIC_FTRACE)
 	select FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY if DYNAMIC_FTRACE
+	select FUNCTION_ALIGNMENT_8B if DYNAMIC_FTRACE_WITH_CALL_OPS
 	select GENERIC_ARCH_TOPOLOGY
 	select GENERIC_ATOMIC64 if !64BIT
 	select GENERIC_CLOCKEVENTS_BROADCAST if SMP
@@ -151,13 +152,15 @@ config RISCV
 	select HAVE_DEBUG_KMEMLEAK
 	select HAVE_DMA_CONTIGUOUS if MMU
 	select HAVE_DYNAMIC_FTRACE if !XIP_KERNEL && MMU && (CLANG_SUPPORTS_DYNAMIC_FTRACE || GCC_SUPPORTS_DYNAMIC_FTRACE)
-	select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+	select FUNCTION_ALIGNMENT_4B if HAVE_DYNAMIC_FTRACE && RISCV_ISA_C
+	select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS if HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS
+	select HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS if (DYNAMIC_FTRACE_WITH_ARGS && !CFI_CLANG)
 	select HAVE_DYNAMIC_FTRACE_WITH_ARGS if HAVE_DYNAMIC_FTRACE
 	select HAVE_FTRACE_GRAPH_FUNC
 	select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
 	select HAVE_FUNCTION_GRAPH_TRACER if HAVE_DYNAMIC_FTRACE_WITH_ARGS
 	select HAVE_FUNCTION_GRAPH_FREGS
-	select HAVE_FUNCTION_TRACER if !XIP_KERNEL && !PREEMPTION
+	select HAVE_FUNCTION_TRACER if !XIP_KERNEL
 	select HAVE_EBPF_JIT if MMU
 	select HAVE_GUP_FAST if MMU
 	select HAVE_FUNCTION_ARG_ACCESS_API
@@ -237,6 +240,7 @@ config CLANG_SUPPORTS_DYNAMIC_FTRACE
 config GCC_SUPPORTS_DYNAMIC_FTRACE
 	def_bool CC_IS_GCC
 	depends on $(cc-option,-fpatchable-function-entry=8)
+	depends on CC_HAS_MIN_FUNCTION_ALIGNMENT || !RISCV_ISA_C
 
 config HAVE_SHADOW_CALL_STACK
 	def_bool $(cc-option,-fsanitize=shadow-call-stack)

arch/riscv/Makefile

@@ -15,9 +15,9 @@ ifeq ($(CONFIG_DYNAMIC_FTRACE),y)
 	LDFLAGS_vmlinux += --no-relax
 	KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
 ifeq ($(CONFIG_RISCV_ISA_C),y)
-	CC_FLAGS_FTRACE := -fpatchable-function-entry=4
+	CC_FLAGS_FTRACE := -fpatchable-function-entry=8,4
 else
-	CC_FLAGS_FTRACE := -fpatchable-function-entry=2
+	CC_FLAGS_FTRACE := -fpatchable-function-entry=4,2
 endif
 endif
 

arch/riscv/include/asm/ftrace.h

@@ -20,10 +20,9 @@ extern void *return_address(unsigned int level);
 #define ftrace_return_address(n) return_address(n)
 
 void _mcount(void);
-static inline unsigned long ftrace_call_adjust(unsigned long addr)
-{
-	return addr;
-}
+unsigned long ftrace_call_adjust(unsigned long addr);
+unsigned long arch_ftrace_get_symaddr(unsigned long fentry_ip);
+#define ftrace_get_symaddr(fentry_ip) arch_ftrace_get_symaddr(fentry_ip)
 
 /*
  * Let's do like x86/arm64 and ignore the compat syscalls.
@@ -57,12 +56,21 @@ struct dyn_arch_ftrace {
  * 2) jalr: setting low-12 offset to ra, jump to ra, and set ra to
  *          return address (original pc + 4)
  *
+ * The first 2 instructions for each tracable function is compiled to 2 nop
+ * instructions. Then, the kernel initializes the first instruction to auipc at
+ * boot time (<ftrace disable>). The second instruction is patched to jalr to
+ * start the trace.
+ *
+ *<Image>:
+ * 0: nop
+ * 4: nop
+ *
  *<ftrace enable>:
- * 0: auipc  t0/ra, 0x?
- * 4: jalr   t0/ra, ?(t0/ra)
+ * 0: auipc  t0, 0x?
+ * 4: jalr   t0, ?(t0)
  *
  *<ftrace disable>:
- * 0: nop
+ * 0: auipc  t0, 0x?
  * 4: nop
  *
  * Dynamic ftrace generates probes to call sites, so we must deal with
@@ -75,10 +83,9 @@ struct dyn_arch_ftrace {
 #define AUIPC_OFFSET_MASK	(0xfffff000)
 #define AUIPC_PAD		(0x00001000)
 #define JALR_SHIFT		20
-#define JALR_RA			(0x000080e7)
-#define AUIPC_RA		(0x00000097)
 #define JALR_T0			(0x000282e7)
 #define AUIPC_T0		(0x00000297)
+#define JALR_RANGE		(JALR_SIGN_MASK - 1)
 
 #define to_jalr_t0(offset)						\
 	(((offset & JALR_OFFSET_MASK) << JALR_SHIFT) | JALR_T0)
@@ -96,26 +103,14 @@ do {									\
 	call[1] = to_jalr_t0(offset);					\
 } while (0)
 
-#define to_jalr_ra(offset)						\
-	(((offset & JALR_OFFSET_MASK) << JALR_SHIFT) | JALR_RA)
-
-#define to_auipc_ra(offset)						\
-	((offset & JALR_SIGN_MASK) ?					\
-	(((offset & AUIPC_OFFSET_MASK) + AUIPC_PAD) | AUIPC_RA) :	\
-	((offset & AUIPC_OFFSET_MASK) | AUIPC_RA))
-
-#define make_call_ra(caller, callee, call)				\
-do {									\
-	unsigned int offset =						\
-		(unsigned long) (callee) - (unsigned long) (caller);	\
-	call[0] = to_auipc_ra(offset);					\
-	call[1] = to_jalr_ra(offset);					\
-} while (0)
-
 /*
- * Let auipc+jalr be the basic *mcount unit*, so we make it 8 bytes here.
+ * Only the jalr insn in the auipc+jalr is patched, so we make it 4
+ * bytes here.
  */
-#define MCOUNT_INSN_SIZE 8
+#define MCOUNT_INSN_SIZE	4
+#define MCOUNT_AUIPC_SIZE	4
+#define MCOUNT_JALR_SIZE	4
+#define MCOUNT_NOP4_SIZE	4
 
 #ifndef __ASSEMBLY__
 struct dyn_ftrace;
@@ -135,6 +130,9 @@ struct __arch_ftrace_regs {
 	unsigned long sp;
 	unsigned long s0;
 	unsigned long t1;
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+	unsigned long direct_tramp;
+#endif
 	union {
 		unsigned long args[8];
 		struct {
@@ -146,6 +144,13 @@ struct __arch_ftrace_regs {
 			unsigned long a5;
 			unsigned long a6;
 			unsigned long a7;
+#ifdef CONFIG_CC_IS_CLANG
+			unsigned long t2;
+			unsigned long t3;
+			unsigned long t4;
+			unsigned long t5;
+			unsigned long t6;
+#endif
 		};
 	};
 };
@@ -221,10 +226,13 @@ void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
 		       struct ftrace_ops *op, struct ftrace_regs *fregs);
 #define ftrace_graph_func ftrace_graph_func
 
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
 static inline void arch_ftrace_set_direct_caller(struct ftrace_regs *fregs, unsigned long addr)
 {
 	arch_ftrace_regs(fregs)->t1 = addr;
 }
+#endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
+
 #endif /* CONFIG_DYNAMIC_FTRACE_WITH_ARGS */
 
 #endif /* __ASSEMBLY__ */

arch/riscv/include/asm/processor.h

@@ -79,6 +79,10 @@ struct pt_regs;
  *       Thus, the task does not own preempt_v. Any use of Vector will have to
  *       save preempt_v, if dirty, and fallback to non-preemptible kernel-mode
  *       Vector.
+ *  - bit 29: The thread voluntarily calls schedule() while holding an active
+ *    preempt_v. All preempt_v context should be dropped in such case because
+ *    V-regs are caller-saved. Only sstatus.VS=ON is persisted across a
+ *    schedule() call.
  *  - bit 30: The in-kernel preempt_v context is saved, and requries to be
  *    restored when returning to the context that owns the preempt_v.
  *  - bit 31: The in-kernel preempt_v context is dirty, as signaled by the
@@ -93,6 +97,7 @@ struct pt_regs;
 #define RISCV_PREEMPT_V			0x00000100
 #define RISCV_PREEMPT_V_DIRTY		0x80000000
 #define RISCV_PREEMPT_V_NEED_RESTORE	0x40000000
+#define RISCV_PREEMPT_V_IN_SCHEDULE	0x20000000
 
 /* CPU-specific state of a task */
 struct thread_struct {

arch/riscv/include/asm/vector.h

@@ -120,6 +120,11 @@ static __always_inline void riscv_v_disable(void)
 		csr_clear(CSR_SSTATUS, SR_VS);
 }
 
+static __always_inline bool riscv_v_is_on(void)
+{
+	return !!(csr_read(CSR_SSTATUS) & SR_VS);
+}
+
 static __always_inline void __vstate_csr_save(struct __riscv_v_ext_state *dest)
 {
 	asm volatile (
@@ -366,6 +371,11 @@ static inline void __switch_to_vector(struct task_struct *prev,
 	struct pt_regs *regs;
 
 	if (riscv_preempt_v_started(prev)) {
+		if (riscv_v_is_on()) {
+			WARN_ON(prev->thread.riscv_v_flags & RISCV_V_CTX_DEPTH_MASK);
+			riscv_v_disable();
+			prev->thread.riscv_v_flags |= RISCV_PREEMPT_V_IN_SCHEDULE;
+		}
 		if (riscv_preempt_v_dirty(prev)) {
 			__riscv_v_vstate_save(&prev->thread.kernel_vstate,
 					      prev->thread.kernel_vstate.datap);
@@ -376,10 +386,16 @@ static inline void __switch_to_vector(struct task_struct *prev,
 		riscv_v_vstate_save(&prev->thread.vstate, regs);
 	}
 
-	if (riscv_preempt_v_started(next))
-		riscv_preempt_v_set_restore(next);
-	else
+	if (riscv_preempt_v_started(next)) {
+		if (next->thread.riscv_v_flags & RISCV_PREEMPT_V_IN_SCHEDULE) {
+			next->thread.riscv_v_flags &= ~RISCV_PREEMPT_V_IN_SCHEDULE;
+			riscv_v_enable();
+		} else {
+			riscv_preempt_v_set_restore(next);
+		}
+	} else {
 		riscv_v_vstate_set_restore(next, task_pt_regs(next));
+	}
 }
 
 void riscv_v_vstate_ctrl_init(struct task_struct *tsk);

arch/riscv/kernel/asm-offsets.c

@@ -493,6 +493,12 @@ void asm_offsets(void)
 	DEFINE(STACKFRAME_SIZE_ON_STACK, ALIGN(sizeof(struct stackframe), STACK_ALIGN));
 	OFFSET(STACKFRAME_FP, stackframe, fp);
 	OFFSET(STACKFRAME_RA, stackframe, ra);
+#ifdef CONFIG_FUNCTION_TRACER
+	DEFINE(FTRACE_OPS_FUNC,		offsetof(struct ftrace_ops, func));
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+	DEFINE(FTRACE_OPS_DIRECT_CALL,	offsetof(struct ftrace_ops, direct_call));
+#endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
+#endif
 
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
 	DEFINE(FREGS_SIZE_ON_STACK, ALIGN(sizeof(struct __arch_ftrace_regs), STACK_ALIGN));
@@ -501,6 +507,13 @@ void asm_offsets(void)
 	DEFINE(FREGS_SP,	    offsetof(struct __arch_ftrace_regs, sp));
 	DEFINE(FREGS_S0,	    offsetof(struct __arch_ftrace_regs, s0));
 	DEFINE(FREGS_T1,	    offsetof(struct __arch_ftrace_regs, t1));
+#ifdef CONFIG_CC_IS_CLANG
+	DEFINE(FREGS_T2,	    offsetof(struct __arch_ftrace_regs, t2));
+	DEFINE(FREGS_T3,	    offsetof(struct __arch_ftrace_regs, t3));
+	DEFINE(FREGS_T4,	    offsetof(struct __arch_ftrace_regs, t4));
+	DEFINE(FREGS_T5,	    offsetof(struct __arch_ftrace_regs, t5));
+	DEFINE(FREGS_T6,	    offsetof(struct __arch_ftrace_regs, t6));
+#endif
 	DEFINE(FREGS_A0,	    offsetof(struct __arch_ftrace_regs, a0));
 	DEFINE(FREGS_A1,	    offsetof(struct __arch_ftrace_regs, a1));
 	DEFINE(FREGS_A2,	    offsetof(struct __arch_ftrace_regs, a2));