powerpc: Rework lazy-interrupt handling

The current implementation of lazy interrupts handling has some
issues that this tries to address.

We don't do the various workarounds we need to do when re-enabling
interrupts in some cases such as when returning from an interrupt
and thus we may still lose or get delayed decrementer or doorbell
interrupts.

The current scheme also makes it much harder to handle the external
"edge" interrupts provided by some BookE processors when using the
EPR facility (External Proxy) and the Freescale Hypervisor.

Additionally, we tend to keep interrupts hard disabled in a number
of cases, such as decrementer interrupts, external interrupts, or
when a masked decrementer interrupt is pending. This is sub-optimal.

This is an attempt at fixing it all in one go by reworking the way
we do the lazy interrupt disabling from the ground up.

The base idea is to replace the "hard_enabled" field with a
"irq_happened" field in which we store a bit mask of what interrupt
occurred while soft-disabled.

When re-enabling, either via arch_local_irq_restore() or when returning
from an interrupt, we can now decide what to do by testing bits in that
field.

We then implement replaying of the missed interrupts either by
re-using the existing exception frame (in exception exit case) or via
the creation of a new one from an assembly trampoline (in the
arch_local_irq_enable case).

This removes the need to play with the decrementer to try to create
fake interrupts, among others.

In addition, this adds a few refinements:

 - We no longer  hard disable decrementer interrupts that occur
while soft-disabled. We now simply bump the decrementer back to max
(on BookS) or leave it stopped (on BookE) and continue with hard interrupts
enabled, which means that we'll potentially get better sample quality from
performance monitor interrupts.

 - Timer, decrementer and doorbell interrupts now hard-enable
shortly after removing the source of the interrupt, which means
they no longer run entirely hard disabled. Again, this will improve
perf sample quality.

 - On Book3E 64-bit, we now make the performance monitor interrupt
act as an NMI like Book3S (the necessary C code for that to work
appear to already be present in the FSL perf code, notably calling
nmi_enter instead of irq_enter). (This also fixes a bug where BookE
perfmon interrupts could clobber r14 ... oops)

 - We could make "masked" decrementer interrupts act as NMIs when doing
timer-based perf sampling to improve the sample quality.

Signed-off-by-yet: Benjamin Herrenschmidt <benh@kernel.crashing.org>
---

v2:

- Add hard-enable to decrementer, timer and doorbells
- Fix CR clobber in masked irq handling on BookE
- Make embedded perf interrupt act as an NMI
- Add a PACA_HAPPENED_EE_EDGE for use by FSL if they want
  to retrigger an interrupt without preventing hard-enable

v3:

 - Fix or vs. ori bug on Book3E
 - Fix enabling of interrupts for some exceptions on Book3E

v4:

 - Fix resend of doorbells on return from interrupt on Book3E

v5:

 - Rebased on top of my latest series, which involves some significant
rework of some aspects of the patch.

v6:
 - 32-bit compile fix
 - more compile fixes with various .config combos
 - factor out the asm code to soft-disable interrupts
 - remove the C wrapper around preempt_schedule_irq

v7:
 - Fix a bug with hard irq state tracking on native power7

1 files changed, 119 insertions, 34 deletions

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index c513beb78b3..f8a7a1a1a9f 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -32,6 +32,7 @@
 #include <asm/ptrace.h>
 #include <asm/irqflags.h>
 #include <asm/ftrace.h>
+#include <asm/hw_irq.h>
 
 /*
  * System calls.
@@ -583,18 +584,72 @@ _GLOBAL(ret_from_except_lite)
         bne     do_work
 #endif /* !CONFIG_PREEMPT */
 
+        .globl  fast_exc_return_irq
+fast_exc_return_irq:
 restore:
+        /*
+         * This is the main kernel exit path, we first check if we
+         * have to change our interrupt state.
+         */
         ld      r5,SOFTE(r1)
-        TRACE_AND_RESTORE_IRQ(r5);
+        lbz     r6,PACASOFTIRQEN(r13)
+        cmpwi   cr1,r5,0
+        cmpw    cr0,r5,r6
+        beq     cr0,4f
+
+        /* We do, handle disable first, which is easy */
+        bne     cr1,3f;
+        li      r0,0
+        stb     r0,PACASOFTIRQEN(r13);
+        TRACE_DISABLE_INTS
+        b       4f
 
-        /* extract EE bit and use it to restore paca->hard_enabled */
-        ld      r3,_MSR(r1)
-        rldicl  r4,r3,49,63             /* r0 = (r3 >> 15) & 1 */
-        stb     r4,PACAHARDIRQEN(r13)
+3:      /*
+         * We are about to soft-enable interrupts (we are hard disabled
+         * at this point). We check if there's anything that needs to
+         * be replayed first.
+         */
+        lbz     r0,PACAIRQHAPPENED(r13)
+        cmpwi   cr0,r0,0
+        bne-    restore_check_irq_replay
+
+        /*
+         * Get here when nothing happened while soft-disabled, just
+         * soft-enable and move-on. We will hard-enable as a side
+         * effect of rfi
+         */
+restore_no_replay:
+        TRACE_ENABLE_INTS
+        li      r0,1
+        stb     r0,PACASOFTIRQEN(r13);
 
+        /*
+         * Final return path. BookE is handled in a different file
+         */
+4:
 #ifdef CONFIG_PPC_BOOK3E
         b       .exception_return_book3e
 #else
+        /*
+         * Clear the reservation. If we know the CPU tracks the address of
+         * the reservation then we can potentially save some cycles and use
+         * a larx. On POWER6 and POWER7 this is significantly faster.
+         */
+BEGIN_FTR_SECTION
+        stdcx.  r0,0,r1         /* to clear the reservation */
+FTR_SECTION_ELSE
+        ldarx   r4,0,r1
+ALT_FTR_SECTION_END_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS)
+
+        /*
+         * Some code path such as load_up_fpu or altivec return directly
+         * here. They run entirely hard disabled and do not alter the
+         * interrupt state. They also don't use lwarx/stwcx. and thus
+         * are known not to leave dangling reservations.
+         */
+        .globl  fast_exception_return
+fast_exception_return:
+        ld      r3,_MSR(r1)
         ld      r4,_CTR(r1)
         ld      r0,_LINK(r1)
         mtctr   r4
@@ -608,17 +663,6 @@ restore:
         beq-    unrecov_restore
 
         /*
-         * Clear the reservation. If we know the CPU tracks the address of
-         * the reservation then we can potentially save some cycles and use
-         * a larx. On POWER6 and POWER7 this is significantly faster.
-         */
-BEGIN_FTR_SECTION
-        stdcx.  r0,0,r1         /* to clear the reservation */
-FTR_SECTION_ELSE
-        ldarx   r4,0,r1
-ALT_FTR_SECTION_END_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS)
-
-        /*
          * Clear RI before restoring r13.  If we are returning to
          * userspace and we take an exception after restoring r13,
          * we end up corrupting the userspace r13 value.
@@ -629,7 +673,8 @@ ALT_FTR_SECTION_END_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS)
 
         /*
          * r13 is our per cpu area, only restore it if we are returning to
-         * userspace
+         * userspace the value stored in the stack frame may belong to
+         * another CPU.
          */
         andi.   r0,r3,MSR_PR
         beq     1f
@@ -654,6 +699,55 @@ ALT_FTR_SECTION_END_IFCLR(CPU_FTR_STCX_CHECKS_ADDRESS)
 
 #endif /* CONFIG_PPC_BOOK3E */
 
+        /*
+         * Something did happen, check if a re-emit is needed
+         * (this also clears paca->irq_happened)
+         */
+restore_check_irq_replay:
+        /* XXX: We could implement a fast path here where we check
+         * for irq_happened being just 0x01, in which case we can
+         * clear it and return. That means that we would potentially
+         * miss a decrementer having wrapped all the way around.
+         *
+         * Still, this might be useful for things like hash_page
+         */
+        bl      .__check_irq_replay
+        cmpwi   cr0,r3,0
+        beq     restore_no_replay
+ 
+        /*
+         * We need to re-emit an interrupt. We do so by re-using our
+         * existing exception frame. We first change the trap value,
+         * but we need to ensure we preserve the low nibble of it
+         */
+        ld      r4,_TRAP(r1)
+        clrldi  r4,r4,60
+        or      r4,r4,r3
+        std     r4,_TRAP(r1)
+
+        /*
+         * Then find the right handler and call it. Interrupts are
+         * still soft-disabled and we keep them that way.
+        */
+        cmpwi   cr0,r3,0x500
+        bne     1f
+        addi    r3,r1,STACK_FRAME_OVERHEAD;
+        bl      .do_IRQ
+        b       .ret_from_except
+1:      cmpwi   cr0,r3,0x900
+        bne     1f
+        addi    r3,r1,STACK_FRAME_OVERHEAD;
+        bl      .timer_interrupt
+        b       .ret_from_except
+#ifdef CONFIG_PPC_BOOK3E
+1:      cmpwi   cr0,r3,0x280
+        bne     1f
+        addi    r3,r1,STACK_FRAME_OVERHEAD;
+        bl      .doorbell_exception
+        b       .ret_from_except
+#endif /* CONFIG_PPC_BOOK3E */
+1:      b       .ret_from_except /* What else to do here ? */
+ 
 do_work:
 #ifdef CONFIG_PREEMPT
         andi.   r0,r3,MSR_PR    /* Returning to user mode? */
@@ -666,18 +760,11 @@ do_work:
         crandc  eq,cr1*4+eq,eq
         bne     restore
 
-        /* Here we are preempting the current task.
-         *
-         * Ensure interrupts are soft-disabled. We also properly mark
-         * the PACA to reflect the fact that they are hard-disabled
-         * and trace the change
+        /*
+         * Here we are preempting the current task. We want to make
+         * sure we are soft-disabled first
          */
-        li      r0,0
-        stb     r0,PACASOFTIRQEN(r13)
-        stb     r0,PACAHARDIRQEN(r13)
-        TRACE_DISABLE_INTS
-
-        /* Call the scheduler with soft IRQs off */
+        SOFT_DISABLE_INTS(r3,r4)
 1:      bl      .preempt_schedule_irq
 
         /* Hard-disable interrupts again (and update PACA) */
@@ -687,8 +774,8 @@ do_work:
         ld      r10,PACAKMSR(r13) /* Get kernel MSR without EE */
         mtmsrd  r10,1
 #endif /* CONFIG_PPC_BOOK3E */
-        li      r0,0
-        stb     r0,PACAHARDIRQEN(r13)
+        li      r0,PACA_IRQ_HARD_DIS
+        stb     r0,PACAIRQHAPPENED(r13)
 
         /* Re-test flags and eventually loop */
         clrrdi  r9,r1,THREAD_SHIFT
@@ -710,14 +797,12 @@ user_work:
 
         andi.   r0,r4,_TIF_NEED_RESCHED
         beq     1f
-        li      r5,1
-        TRACE_AND_RESTORE_IRQ(r5);
+        bl      .restore_interrupts
         bl      .schedule
         b       .ret_from_except_lite
 
 1:      bl      .save_nvgprs
-        li      r5,1
-        TRACE_AND_RESTORE_IRQ(r5);
+        bl      .restore_interrupts
         addi    r3,r1,STACK_FRAME_OVERHEAD
         bl      .do_notify_resume
         b       .ret_from_except