[PATCH v3 10/15] target/ppc/excp_helper.c: EBB handling adjustments

[Daniel Henrique Barboza <danielhb413@gmail.com>]

The current logic is only considering event-based exceptions triggered
by the performance monitor. This is true now, but we might want to add
support for external event-based exceptions in the future.

Let's make it a bit easier to do so by adding the bit logic that would
happen in case we were dealing with an external event-based exception.

While we're at it, add a few comments explaining why we're setting and
clearing BESCR bits.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
---
 target/ppc/excp_helper.c | 45 ++++++++++++++++++++++++++++++++++------
 1 file changed, 39 insertions(+), 6 deletions(-)

diff --git a/target/ppc/excp_helper.c b/target/ppc/excp_helper.c
index d20447f171..6d16c986af 100644
--- a/target/ppc/excp_helper.c
+++ b/target/ppc/excp_helper.c
@@ -822,14 +822,47 @@ static inline void powerpc_excp(PowerPCCPU *cpu, int excp_model, int excp)
                   "is not implemented yet !\n");
         break;
     case POWERPC_EXCP_EBB:       /* Event-based branch exception             */
-        if ((env->spr[SPR_BESCR] & BESCR_GE) &&
-            (env->spr[SPR_BESCR] & BESCR_PME)) {
+        if (env->spr[SPR_BESCR] & BESCR_GE) {
             target_ulong nip;
 
-            env->spr[SPR_BESCR] &= ~BESCR_GE;   /* Clear GE */
-            env->spr[SPR_BESCR] |= BESCR_PMEO;  /* Set PMEO */
-            env->spr[SPR_EBBRR] = env->nip;     /* Save NIP for rfebb insn */
-            nip = env->spr[SPR_EBBHR];          /* EBB handler */
+            /*
+             * If we have Performance Monitor Event-Based exception
+             * enabled (BESCR_PME) and a Performance Monitor alert
+             * occurred (MMCR0_PMAO), clear BESCR_PME and set BESCR_PMEO
+             * (Performance Monitor Event-Based Exception Occurred).
+             *
+             * Software is responsible for clearing both BESCR_PMEO and
+             * MMCR0_PMAO after the event has been handled.
+             */
+            if ((env->spr[SPR_BESCR] & BESCR_PME) &&
+                (env->spr[SPR_POWER_MMCR0] & MMCR0_PMAO)) {
+                env->spr[SPR_BESCR] &= ~BESCR_PME;
+                env->spr[SPR_BESCR] |= BESCR_PMEO;
+            }
+
+            /*
+             * In the case of External Event-Based exceptions, do a
+             * similar logic with BESCR_EE and BESCR_EEO. BESCR_EEO must
+             * also be cleared by software.
+             *
+             * PowerISA 3.1 considers that we'll not have BESCR_PMEO and
+             * BESCR_EEO set at the same time. We can check for BESCR_PMEO
+             * being not set in step above to see if this exception was
+             * trigged by an external event.
+             */
+            if (env->spr[SPR_BESCR] & BESCR_EE &&
+                !(env->spr[SPR_BESCR] & BESCR_PMEO)) {
+                env->spr[SPR_BESCR] &= ~BESCR_EE;
+                env->spr[SPR_BESCR] |= BESCR_EEO;
+            }
+
+            /*
+             * Clear BESCR_GE, save NIP for 'rfebb' and point the
+             * execution to the event handler (SPR_EBBHR) address.
+             */
+            env->spr[SPR_BESCR] &= ~BESCR_GE;
+            env->spr[SPR_EBBRR] = env->nip;
+            nip = env->spr[SPR_EBBHR];
             powerpc_set_excp_state(cpu, nip, env->msr);
         }
         /*
-- 
2.31.1