[RFC 09/12] riscv: Enable NMIs during exceptions

[Xu Lu]

We have switched the way of disabling irqs to CSR_IE masking. But
hardware still automatically clearing SIE field of CSR_STATUS whenever
thread traps into kernel, which disabling all irqs including NMIs.

This commit re-enables NMIs and normal irqs during exceptions by setting
the SIE field in CSR_STATUS and restoring NMI and irq bits in CSR_IE.

Signed-off-by: Xu Lu <luxu.kernel at bytedance.com>
---
 arch/riscv/include/asm/irqflags.h  | 13 +++++++++++++
 arch/riscv/include/asm/switch_to.h |  7 +++++++
 arch/riscv/kernel/traps.c          | 10 ++++++++++
 3 files changed, 30 insertions(+)

diff --git a/arch/riscv/include/asm/irqflags.h b/arch/riscv/include/asm/irqflags.h
index 42f7803582df..6a709e9c69ca 100644
--- a/arch/riscv/include/asm/irqflags.h
+++ b/arch/riscv/include/asm/irqflags.h
@@ -29,6 +29,16 @@ static inline void set_nmi(int irq) {}
 
 static inline void unset_nmi(int irq) {}
 
+static inline void enable_nmis(void)
+{
+	csr_set(CSR_IE, ALLOWED_NMI_MASK);
+}
+
+static inline void disable_nmis(void)
+{
+	csr_clear(CSR_IE, ALLOWED_NMI_MASK);
+}
+
 static inline void local_irq_switch_on(void)
 {
 	csr_set(CSR_STATUS, SR_IE);
@@ -128,6 +138,9 @@ static inline void arch_local_irq_restore(unsigned long flags)
 	csr_set(CSR_STATUS, flags & SR_IE);
 }
 
+static inline void enable_nmis(void) {}
+static inline void disable_nmis(void) {}
+
 #endif /* !CONFIG_RISCV_PSEUDO_NMI */
 
 #endif /* _ASM_RISCV_IRQFLAGS_H */
diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h
index a727be723c56..116cffeaa6bf 100644
--- a/arch/riscv/include/asm/switch_to.h
+++ b/arch/riscv/include/asm/switch_to.h
@@ -84,4 +84,11 @@ do {							\
 	((last) = __switch_to(__prev, __next));		\
 } while (0)
 
+#ifdef CONFIG_RISCV_PSEUDO_NMI
+
+#define prepare_arch_switch(next)			disable_nmis()
+#define finish_arch_post_lock_switch()			enable_nmis()
+
+#endif /* CONFIG_RISCV_PSEUDO_NMI */
+
 #endif /* _ASM_RISCV_SWITCH_TO_H */
diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index fae8f610d867..63d3c1417563 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -135,7 +135,9 @@ asmlinkage __visible __trap_section void name(struct pt_regs *regs)		\
 {										\
 	if (user_mode(regs)) {							\
 		irqentry_enter_from_user_mode(regs);				\
+		enable_nmis();							\
 		do_trap_error(regs, signo, code, regs->epc, "Oops - " str);	\
+		disable_nmis();							\
 		irqentry_exit_to_user_mode(regs);				\
 	} else {								\
 		irqentry_state_t state = irqentry_nmi_enter(regs);		\
@@ -292,8 +294,12 @@ asmlinkage __visible __trap_section void do_trap_break(struct pt_regs *regs)
 	if (user_mode(regs)) {
 		irqentry_enter_from_user_mode(regs);
 
+		enable_nmis();
+
 		handle_break(regs);
 
+		disable_nmis();
+
 		irqentry_exit_to_user_mode(regs);
 	} else {
 		irqentry_state_t state = irqentry_nmi_enter(regs);
@@ -338,10 +344,14 @@ asmlinkage __visible noinstr void do_page_fault(struct pt_regs *regs)
 {
 	irqentry_state_t state = irqentry_enter(regs);
 
+	enable_nmis();
+
 	handle_page_fault(regs);
 
 	local_irq_disable();
 
+	disable_nmis();
+
 	irqentry_exit(regs, state);
 }
 #endif
-- 
2.20.1