[PATCH v4] riscv: add irq stack support

[Jisheng Zhang]

Currently, IRQs are still handled on the kernel stack of the current
task on riscv platforms. If the task has a deep call stack at the time
of interrupt, and handling the interrupt also requires a deep stack,
it's possible to see stack overflow.

Before this patch, the stack_max_size of a v5.17-rc1 kernel running on
a lichee RV board gave:
~ # cat /sys/kernel/debug/tracing/stack_max_size
3736

After this patch,
~ # cat /sys/kernel/debug/tracing/stack_max_size
3176

We reduce the max kernel stack usage by 560 bytes!

Signed-off-by: Jisheng Zhang <jszhang at kernel.org>
---
since v3:
 - Always use vmalloc to allocate irq stack. Thank Arnd.

since v2:
 - rebase on v5.18-rcN
 - update commit msg, I.E remove the "it's possible to reduce the
THREAD_SIZE to 8KB for RV64 platforms..."

since v1:
 - add __ro_after_init to the irq_stack[] array.

 arch/riscv/include/asm/thread_info.h |  1 +
 arch/riscv/kernel/asm-offsets.c      |  2 ++
 arch/riscv/kernel/entry.S            | 33 +++++++++++++++++++++++++---
 arch/riscv/kernel/irq.c              | 12 ++++++++++
 4 files changed, 45 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
index 74d888c8d631..98ea73721a0b 100644
--- a/arch/riscv/include/asm/thread_info.h
+++ b/arch/riscv/include/asm/thread_info.h
@@ -25,6 +25,7 @@
 #endif
 #define THREAD_SIZE		(PAGE_SIZE << THREAD_SIZE_ORDER)
 
+#define IRQ_STACK_SIZE		THREAD_SIZE
 /*
  * By aligning VMAP'd stacks to 2 * THREAD_SIZE, we can detect overflow by
  * checking sp & (1 << THREAD_SHIFT), which we can do cheaply in the entry
diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index df9444397908..9e32748af0e8 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -37,6 +37,8 @@ void asm_offsets(void)
 	OFFSET(TASK_TI_PREEMPT_COUNT, task_struct, thread_info.preempt_count);
 	OFFSET(TASK_TI_KERNEL_SP, task_struct, thread_info.kernel_sp);
 	OFFSET(TASK_TI_USER_SP, task_struct, thread_info.user_sp);
+	OFFSET(TASK_TI_CPU, task_struct, thread_info.cpu);
+	OFFSET(TASK_STACK, task_struct, stack);
 
 	OFFSET(TASK_THREAD_F0,  task_struct, thread.fstate.f[0]);
 	OFFSET(TASK_THREAD_F1,  task_struct, thread.fstate.f[1]);
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index c8b9ce274b9a..e91cae183ef4 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -126,12 +126,39 @@ skip_context_tracking:
 	 */
 	bge s4, zero, 1f
 
-	la ra, ret_from_exception
+	/* preserve the sp */
+	move s0, sp
 
-	/* Handle interrupts */
 	move a0, sp /* pt_regs */
+
+	/*
+	 * Compare sp with the base of the task stack.
+	 * If the top ~(THREAD_SIZE - 1) bits match, we are on a task stack,
+	 * and should switch to the irq stack.
+	 */
+	REG_L t0, TASK_STACK(tp)
+	xor t0, t0, s0
+	li t1, ~(THREAD_SIZE - 1)
+	and t0, t0, t1
+	bnez t0, 2f
+
+	la t1, irq_stack
+	REG_L t2, TASK_TI_CPU(tp)
+	slli t2, t2, RISCV_LGPTR
+	add t1, t1, t2
+	REG_L t2, 0(t1)
+	li t1, IRQ_STACK_SIZE
+	/* switch to the irq stack */
+	add sp, t2, t1
+
+2:
+	/* Handle interrupts */
 	la a1, generic_handle_arch_irq
-	jr a1
+	jalr a1
+
+	/* Restore sp */
+	move sp, s0
+	j ret_from_exception
 1:
 	/*
 	 * Exceptions run with interrupts enabled or disabled depending on the
diff --git a/arch/riscv/kernel/irq.c b/arch/riscv/kernel/irq.c
index 7207fa08d78f..6b26960e2e17 100644
--- a/arch/riscv/kernel/irq.c
+++ b/arch/riscv/kernel/irq.c
@@ -10,6 +10,8 @@
 #include <linux/seq_file.h>
 #include <asm/smp.h>
 
+void *irq_stack[NR_CPUS] __ro_after_init;
+
 int arch_show_interrupts(struct seq_file *p, int prec)
 {
 	show_ipi_stats(p, prec);
@@ -18,7 +20,17 @@ int arch_show_interrupts(struct seq_file *p, int prec)
 
 void __init init_IRQ(void)
 {
+	int cpu;
+
 	irqchip_init();
 	if (!handle_arch_irq)
 		panic("No interrupt controller found.");
+
+	for_each_possible_cpu(cpu) {
+		void *s = __vmalloc_node(IRQ_STACK_SIZE, THREAD_ALIGN,
+					 THREADINFO_GFP, cpu_to_node(cpu),
+					 __builtin_return_address(0));
+
+		irq_stack[cpu] = s;
+	}
 }
-- 
2.34.1