Fix wakeup latency when sleeping with 'wait'

If we handle an interrupt just before the 'wait' and the interrupt
schedules some work, we need to skip the 'wait' call. The simple solution
of calling sched_runnable() with interrupts disabled immediately before
wait still leaves a window after the call and before 'wait' in which
the same issue can occur.

The solution implemented is to check the EPC in the interrupt handler, and
if it is in a region before the 'wait' call, to fix up the EPC to skip the
wait call.

Reported/analysed by:	adrian
Fix suggested by:	kib

Reviewed by:	jmallett, imp

sys/mips/include/md_var.h

@@ -56,6 +56,7 @@ void MipsSwitchFPState(struct thread *, struct trapframe *);
 u_long	kvtop(void *addr);
 int	is_cacheable_mem(vm_paddr_t addr);
 void	mips_generic_reset(void);
+void	mips_wait(void);
 
 #define	MIPS_DEBUG   0
 

sys/mips/mips/exception.S

@@ -557,6 +557,33 @@ NNON_LEAF(MipsUserGenException, CALLFRAME_SIZ, ra)
 	.set	at
 END(MipsUserGenException)
 
+	.set	push
+	.set	noat
+NON_LEAF(mips_wait, CALLFRAME_SIZ, ra)
+	PTR_SUBU        sp, sp, CALLFRAME_SIZ
+	.mask   0x80000000, (CALLFRAME_RA - CALLFRAME_SIZ)
+	REG_S   ra, CALLFRAME_RA(sp)		# save RA
+	mfc0	t0, MIPS_COP_0_STATUS
+	xori	t1, t0, MIPS_SR_INT_IE
+	mtc0	t1, MIPS_COP_0_STATUS
+	COP0_SYNC
+	jal	sched_runnable
+	nop
+	REG_L   ra, CALLFRAME_RA(sp)
+	mfc0	t0, MIPS_COP_0_STATUS
+	ori	t1, t0, MIPS_SR_INT_IE
+	.align 4
+GLOBAL(MipsWaitStart)			# this is 16 byte aligned
+	mtc0	t1, MIPS_COP_0_STATUS
+	bnez	v0, MipsWaitEnd
+	nop
+	wait
+GLOBAL(MipsWaitEnd)			# MipsWaitStart + 16
+	jr	ra
+	PTR_ADDU        sp, sp, CALLFRAME_SIZ
+END(mips_wait)
+	.set	pop
+
 /*----------------------------------------------------------------------------
  *
  * MipsKernIntr --
@@ -578,6 +605,19 @@ NNON_LEAF(MipsKernIntr, KERN_EXC_FRAME_SIZE, ra)
 	.set	noat
 	PTR_SUBU	sp, sp, KERN_EXC_FRAME_SIZE
 	.mask	0x80000000, (CALLFRAME_RA - KERN_EXC_FRAME_SIZE)
+
+/*
+ * Check for getting interrupts just before wait
+ */
+	MFC0	k0, MIPS_COP_0_EXC_PC
+	ori	k0, 0xf
+	xori	k0, 0xf			# 16 byte align
+	PTR_LA	k1, MipsWaitStart
+	bne	k0, k1, 1f
+	nop
+	PTR_ADDU k1, 16			# skip over wait
+	MTC0	k1, MIPS_COP_0_EXC_PC
+1:
 /*
  *  Save CPU state, building 'frame'.
  */

sys/mips/mips/machdep.c

@@ -163,6 +163,9 @@ extern char MipsTLBMiss[], MipsTLBMissEnd[];
 /* Cache error handler */
 extern char MipsCache[], MipsCacheEnd[];
 
+/* MIPS wait skip region */
+extern char MipsWaitStart[], MipsWaitEnd[];
+
 extern char edata[], end[];
 #ifdef DDB
 extern vm_offset_t ksym_start, ksym_end;
@@ -326,6 +329,12 @@ struct msgbuf *msgbufp=0;
 void
 mips_vector_init(void)
 {
+	/*
+	 * Make sure that the Wait region logic is not been 
+	 * changed
+	 */
+	if (MipsWaitEnd - MipsWaitStart != 16)
+		panic("startup: MIPS wait region not correct");
 	/*
 	 * Copy down exception vector code.
 	 */
@@ -485,24 +494,9 @@ spinlock_exit(void)
 /*
  * call platform specific code to halt (until next interrupt) for the idle loop
  */
-/*
- * This is disabled because of three issues:
- *
- * + By calling critical_enter(), any interrupt which occurs after that but
- *   before the wait instruction will be handled but not serviced (in the case
- *   of a netisr) because preemption is not allowed at this point;
- * + Any fast interrupt handler which schedules an immediate or fast callout
- *   will not occur until the wait instruction is interrupted, as the clock
- *   has already been set by cpu_idleclock();
- * + There is currently no known way to atomically enable interrupts and call
- *   wait, which is how the i386/amd64 code gets around (1). Thus even if
- *   interrupts were disabled and reenabled just before the wait call, any
- *   interrupt that did occur may not interrupt wait.
- */
 void
 cpu_idle(int busy)
 {
-#if 0
 	KASSERT((mips_rd_status() & MIPS_SR_INT_IE) != 0,
 		("interrupts disabled in idle process."));
 	KASSERT((mips_rd_status() & MIPS_INT_MASK) != 0,
@@ -512,12 +506,11 @@ cpu_idle(int busy)
 		critical_enter();
 		cpu_idleclock();
 	}
-	__asm __volatile ("wait");
+	mips_wait();
 	if (!busy) {
 		cpu_activeclock();
 		critical_exit();
 	}
-#endif	
 }
 
 int