[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>

1 files changed, 7 insertions, 6 deletions

diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
index 11b4f6d9ffce..d3aee08e6814 100644
--- a/arch/powerpc/kernel/head_64.S
+++ b/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