[PATCH v2] 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!

>From another side, after this patch, it's possible to reduce the
THREAD_SIZE to 8KB for RV64 platforms. This is especially useful for
those systems with small memory size, e.g the Allwinner D1S platform
which is RV64 but only has 64MB DDR.

Signed-off-by: Jisheng Zhang <jszhang at kernel.org>
---
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              | 16 ++++++++++++++
 4 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
index 60da0dcacf14..67387a8bcb34 100644
--- a/arch/riscv/include/asm/thread_info.h
+++ b/arch/riscv/include/asm/thread_info.h
@@ -19,6 +19,7 @@
 #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 df0519a64eaf..9619398a69e1 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -36,6 +36,8 @@ 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 ed29e9c8f660..57c9b64e16a5 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..f20cbfd42e82 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 +20,21 @@ 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) {
+#ifdef CONFIG_VMAP_STACK
+		void *s = __vmalloc_node(IRQ_STACK_SIZE, THREAD_ALIGN,
+					 THREADINFO_GFP, cpu_to_node(cpu),
+					 __builtin_return_address(0));
+#else
+		void *s = (void *)__get_free_pages(GFP_KERNEL, get_order(IRQ_STACK_SIZE));
+#endif
+
+		irq_stack[cpu] = s;
+	}
 }
-- 
2.34.1