LoongArch: Fix idle VS timer enqueue

LoongArch re-enables interrupts on its idle routine and performs a
TIF_NEED_RESCHED check afterwards before putting the CPU to sleep.

The IRQs firing between the check and the idle instruction may set the
TIF_NEED_RESCHED flag. In order to deal with such a race, IRQs
interrupting __arch_cpu_idle() rollback their return address to the
beginning of __arch_cpu_idle() so that TIF_NEED_RESCHED is checked
again before going back to sleep.

However idle IRQs can also queue timers that may require a tick
reprogramming through a new generic idle loop iteration but those timers
would go unnoticed here because __arch_cpu_idle() only checks
TIF_NEED_RESCHED. It doesn't check for pending timers.

Fix this with fast-forwarding idle IRQs return address to the end of the
idle routine instead of the beginning, so that the generic idle loop can
handle both TIF_NEED_RESCHED and pending timers.

Fixes: 0603839 ("LoongArch: Add exception/interrupt handling")
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Frederic Weisbecker <[email protected]>
Signed-off-by: Marco Crivellari <[email protected]>
Signed-off-by: Huacai Chen <[email protected]>

Author: DispatchCode

arch/loongarch/kernel/genex.S

@@ -18,28 +18,30 @@
 
 	.align	5
 SYM_FUNC_START(__arch_cpu_idle)
-	/* start of rollback region */
-	LONG_L	t0, tp, TI_FLAGS
-	nop
-	andi	t0, t0, _TIF_NEED_RESCHED
-	bnez	t0, 1f
-	nop
-	nop
-	nop
+	/* start of idle interrupt region */
+	ori	t0, zero, CSR_CRMD_IE
+	/* idle instruction needs irq enabled */
+	csrxchg	t0, t0, LOONGARCH_CSR_CRMD
+	/*
+	 * If an interrupt lands here; between enabling interrupts above and
+	 * going idle on the next instruction, we must *NOT* go idle since the
+	 * interrupt could have set TIF_NEED_RESCHED or caused an timer to need
+	 * reprogramming. Fall through -- see handle_vint() below -- and have
+	 * the idle loop take care of things.
+	 */
 	idle	0
-	/* end of rollback region */
+	/* end of idle interrupt region */
 1:	jr	ra
 SYM_FUNC_END(__arch_cpu_idle)
 
 SYM_CODE_START(handle_vint)
 	UNWIND_HINT_UNDEFINED
 	BACKUP_T0T1
 	SAVE_ALL
-	la_abs	t1, __arch_cpu_idle
+	la_abs	t1, 1b
 	LONG_L	t0, sp, PT_ERA
-	/* 32 byte rollback region */
-	ori	t0, t0, 0x1f
-	xori	t0, t0, 0x1f
+	/* 3 instructions idle interrupt region */
+	ori	t0, t0, 0b1100
 	bne	t0, t1, 1f
 	LONG_S	t0, sp, PT_ERA
 1:	move	a0, sp

arch/loongarch/kernel/idle.c

@@ -11,7 +11,6 @@
 
 void __cpuidle arch_cpu_idle(void)
 {
-	raw_local_irq_enable();
-	__arch_cpu_idle(); /* idle instruction needs irq enabled */
+	__arch_cpu_idle();
 	raw_local_irq_disable();
 }

arch/loongarch/kernel/reset.c

@@ -33,7 +33,7 @@ void machine_halt(void)
 	console_flush_on_panic(CONSOLE_FLUSH_PENDING);
 
 	while (true) {
-		__arch_cpu_idle();
+		__asm__ __volatile__("idle 0" : : : "memory");
 	}
 }
 
@@ -53,7 +53,7 @@ void machine_power_off(void)
 #endif
 
 	while (true) {
-		__arch_cpu_idle();
+		__asm__ __volatile__("idle 0" : : : "memory");
 	}
 }
 
@@ -74,6 +74,6 @@ void machine_restart(char *command)
 		acpi_reboot();
 
 	while (true) {
-		__arch_cpu_idle();
+		__asm__ __volatile__("idle 0" : : : "memory");
 	}
 }