[POWERPC] Fix iSeries hard irq enabling regression

A subtle bug sneaked into iSeries recently.  On this platform, we must
not normally clear MSR:EE (the hardware external interrupt enable)
except for short periods of time.  Taking an interrupt while
soft-disabled doesn't cause us to clear it for example.

The iSeries kernel expects to mostly run with MSR:EE enabled at all
times except in a few exception entry/exit code paths.  Thus
local_irq_enable() doesn't check if it needs to hard-enable as it
expects this to be unnecessary on iSeries.

However, hard_irq_disable() _does_ cause MSR:EE to be cleared,
including on iSeries.  A call to it was recently added to the
context switch code, thus causing interrupts to become disabled
for a long periods of time, causing the iSeries watchdog to kick
in under some circumstances and other nasty things.

This patch fixes it by making local_irq_enable() properly re-enable
MSR:EE on iSeries.  It basically removes a return statement here
to make iSeries use the same code path as everybody else.  That does
mean that we might occasionally get spurious decrementer interrupts
but I don't think that matters.

Another option would have been to make hard_irq_disable() a nop
on iSeries but I didn't like it much, in case we have good reasons
to hard-disable.

Part of the patch is fixes to make sure the hard_enabled PACA field
is properly set on iSeries as it used not to be before, since it
was mostly unused.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/head_64.S

@@ -1387,12 +1387,14 @@ __secondary_start:
 #ifdef CONFIG_PPC_ISERIES
 BEGIN_FW_FTR_SECTION
 	ori	r4,r4,MSR_EE
+	li	r8,1
+	stb	r8,PACAHARDIRQEN(r13)
 END_FW_FTR_SECTION_IFSET(FW_FEATURE_ISERIES)
 #endif
 BEGIN_FW_FTR_SECTION
-	stb	r7,PACASOFTIRQEN(r13)
 	stb	r7,PACAHARDIRQEN(r13)
 END_FW_FTR_SECTION_IFCLR(FW_FEATURE_ISERIES)
+	stb	r7,PACASOFTIRQEN(r13)
 
 	mtspr	SPRN_SRR0,r3
 	mtspr	SPRN_SRR1,r4
@@ -1520,15 +1522,14 @@ _INIT_GLOBAL(start_here_common)
 #ifdef CONFIG_PPC_ISERIES
 BEGIN_FW_FTR_SECTION
 	mfmsr	r5
-	ori	r5,r5,MSR_EE		/* Hard Enabled */
+	ori	r5,r5,MSR_EE		/* Hard Enabled on iSeries*/
 	mtmsrd	r5
+	li	r5,1
 END_FW_FTR_SECTION_IFSET(FW_FEATURE_ISERIES)
 #endif
-BEGIN_FW_FTR_SECTION
-	stb	r5,PACAHARDIRQEN(r13)
-END_FW_FTR_SECTION_IFCLR(FW_FEATURE_ISERIES)
+	stb	r5,PACAHARDIRQEN(r13)	/* Hard Disabled on others */
 
-	bl .start_kernel
+	bl	.start_kernel
 
 	/* Not reached */
 	BUG_OPCODE

arch/powerpc/kernel/irq.c

@@ -143,7 +143,6 @@ void local_irq_restore(unsigned long en)
 		 */
 		if (local_paca->lppaca_ptr->int_dword.any_int)
 			iseries_handle_interrupts();
-		return;
 	}
 
 	/*