riscv: Deduplicate IRQ stack switching

With CONFIG_IRQ_STACKS, we switch to a separate per-CPU IRQ stack
before calling handle_riscv_irq or __do_softirq. We currently
have duplicate inline assembly snippets for stack switching in
both code paths. Now that we can access per-CPU variables in
assembly, implement call_on_irq_stack in assembly, and use that
instead of redundant inline assembly.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20230927224757.1154247-10-samitolvanen@google.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: samitolvanen

arch/riscv/include/asm/asm.h

@@ -104,6 +104,11 @@
 .endm
 #endif /* CONFIG_SMP */
 
+.macro load_per_cpu dst ptr tmp
+	asm_per_cpu \dst \ptr \tmp
+	REG_L \dst, 0(\dst)
+.endm
+
 	/* save all GPs except x1 ~ x5 */
 	.macro save_from_x6_to_x31
 	REG_S x6,  PT_T1(sp)

arch/riscv/include/asm/irq_stack.h

@@ -12,6 +12,9 @@
 
 DECLARE_PER_CPU(ulong *, irq_stack_ptr);
 
+asmlinkage void call_on_irq_stack(struct pt_regs *regs,
+				  void (*func)(struct pt_regs *));
+
 #ifdef CONFIG_VMAP_STACK
 /*
  * To ensure that VMAP'd stack overflow detection works correctly, all VMAP'd

arch/riscv/kernel/asm-offsets.c

@@ -14,6 +14,7 @@
 #include <asm/thread_info.h>
 #include <asm/ptrace.h>
 #include <asm/cpu_ops_sbi.h>
+#include <asm/stacktrace.h>
 #include <asm/suspend.h>
 
 void asm_offsets(void);
@@ -480,4 +481,8 @@ void asm_offsets(void)
 	OFFSET(KERNEL_MAP_VIRT_ADDR, kernel_mapping, virt_addr);
 	OFFSET(SBI_HART_BOOT_TASK_PTR_OFFSET, sbi_hart_boot_data, task_ptr);
 	OFFSET(SBI_HART_BOOT_STACK_PTR_OFFSET, sbi_hart_boot_data, stack_ptr);
+
+	DEFINE(STACKFRAME_SIZE_ON_STACK, ALIGN(sizeof(struct stackframe), STACK_ALIGN));
+	OFFSET(STACKFRAME_FP, stackframe, fp);
+	OFFSET(STACKFRAME_RA, stackframe, ra);
 }

arch/riscv/kernel/entry.S

@@ -218,6 +218,36 @@ SYM_CODE_START(ret_from_fork)
 	tail syscall_exit_to_user_mode
 SYM_CODE_END(ret_from_fork)
 
+#ifdef CONFIG_IRQ_STACKS
+/*
+ * void call_on_irq_stack(struct pt_regs *regs,
+ * 		          void (*func)(struct pt_regs *));
+ *
+ * Calls func(regs) using the per-CPU IRQ stack.
+ */
+SYM_FUNC_START(call_on_irq_stack)
+	/* Create a frame record to save ra and s0 (fp) */
+	addi	sp, sp, -STACKFRAME_SIZE_ON_STACK
+	REG_S	ra, STACKFRAME_RA(sp)
+	REG_S	s0, STACKFRAME_FP(sp)
+	addi	s0, sp, STACKFRAME_SIZE_ON_STACK
+
+	/* Switch to the per-CPU IRQ stack and call the handler */
+	load_per_cpu t0, irq_stack_ptr, t1
+	li	t1, IRQ_STACK_SIZE
+	add	sp, t0, t1
+	jalr	a1
+
+	/* Switch back to the thread stack and restore ra and s0 */
+	addi	sp, s0, -STACKFRAME_SIZE_ON_STACK
+	REG_L	ra, STACKFRAME_RA(sp)
+	REG_L	s0, STACKFRAME_FP(sp)
+	addi	sp, sp, STACKFRAME_SIZE_ON_STACK
+
+	ret
+SYM_FUNC_END(call_on_irq_stack)
+#endif /* CONFIG_IRQ_STACKS */
+
 /*
  * Integer register context switch
  * The callee-saved registers must be saved and restored.

arch/riscv/kernel/irq.c

@@ -61,35 +61,16 @@ static void init_irq_stacks(void)
 #endif /* CONFIG_VMAP_STACK */
 
 #ifdef CONFIG_HAVE_SOFTIRQ_ON_OWN_STACK
+static void ___do_softirq(struct pt_regs *regs)
+{
+	__do_softirq();
+}
+
 void do_softirq_own_stack(void)
 {
-#ifdef CONFIG_IRQ_STACKS
-	if (on_thread_stack()) {
-		ulong *sp = per_cpu(irq_stack_ptr, smp_processor_id())
-					+ IRQ_STACK_SIZE/sizeof(ulong);
-		__asm__ __volatile(
-		"addi	sp, sp, -"RISCV_SZPTR  "\n"
-		REG_S"  ra, (sp)		\n"
-		"addi	sp, sp, -"RISCV_SZPTR  "\n"
-		REG_S"  s0, (sp)		\n"
-		"addi	s0, sp, 2*"RISCV_SZPTR "\n"
-		"move	sp, %[sp]		\n"
-		"call	__do_softirq		\n"
-		"addi	sp, s0, -2*"RISCV_SZPTR"\n"
-		REG_L"  s0, (sp)		\n"
-		"addi	sp, sp, "RISCV_SZPTR   "\n"
-		REG_L"  ra, (sp)		\n"
-		"addi	sp, sp, "RISCV_SZPTR   "\n"
-		:
-		: [sp] "r" (sp)
-		: "a0", "a1", "a2", "a3", "a4", "a5", "a6", "a7",
-		  "t0", "t1", "t2", "t3", "t4", "t5", "t6",
-#ifndef CONFIG_FRAME_POINTER
-		  "s0",
-#endif
-		  "memory");
-	} else
-#endif
+	if (on_thread_stack())
+		call_on_irq_stack(NULL, ___do_softirq);
+	else
 		__do_softirq();
 }
 #endif /* CONFIG_HAVE_SOFTIRQ_ON_OWN_STACK */

arch/riscv/kernel/traps.c

@@ -352,34 +352,10 @@ static void noinstr handle_riscv_irq(struct pt_regs *regs)
 asmlinkage void noinstr do_irq(struct pt_regs *regs)
 {
 	irqentry_state_t state = irqentry_enter(regs);
-#ifdef CONFIG_IRQ_STACKS
-	if (on_thread_stack()) {
-		ulong *sp = per_cpu(irq_stack_ptr, smp_processor_id())
-					+ IRQ_STACK_SIZE/sizeof(ulong);
-		__asm__ __volatile(
-		"addi	sp, sp, -"RISCV_SZPTR  "\n"
-		REG_S"  ra, (sp)		\n"
-		"addi	sp, sp, -"RISCV_SZPTR  "\n"
-		REG_S"  s0, (sp)		\n"
-		"addi	s0, sp, 2*"RISCV_SZPTR "\n"
-		"move	sp, %[sp]		\n"
-		"move	a0, %[regs]		\n"
-		"call	handle_riscv_irq	\n"
-		"addi	sp, s0, -2*"RISCV_SZPTR"\n"
-		REG_L"  s0, (sp)		\n"
-		"addi	sp, sp, "RISCV_SZPTR   "\n"
-		REG_L"  ra, (sp)		\n"
-		"addi	sp, sp, "RISCV_SZPTR   "\n"
-		:
-		: [sp] "r" (sp), [regs] "r" (regs)
-		: "a0", "a1", "a2", "a3", "a4", "a5", "a6", "a7",
-		  "t0", "t1", "t2", "t3", "t4", "t5", "t6",
-#ifndef CONFIG_FRAME_POINTER
-		  "s0",
-#endif
-		  "memory");
-	} else
-#endif
+
+	if (IS_ENABLED(CONFIG_IRQ_STACKS) && on_thread_stack())
+		call_on_irq_stack(regs, handle_riscv_irq);
+	else
 		handle_riscv_irq(regs);
 
 	irqentry_exit(regs, state);