PPC: Clean up DECR implementation

There are 3 different variants of the decrementor for BookE and BookS.

The BookE variant sets TSR[DIS] to 1 when the DEC value becomes 1 or 0. TSR[DIS]
is then the indicator whether the decrementor interrupt line is asserted or not.

The old BookS variant treats DEC as an edge interrupt that gets triggered when
the DEC value's top bit turns 1 from 0.

The new BookS variant maintains the assertion bit inside DEC itself. Whenever
the DEC value becomes negative (top bit set) the DEC interrupt line is asserted.

So far we implemented mostly the old BookS variant. Let's do them all properly.

This fixes booting pseries ppc64 guest images in TCG mode for me.

Signed-off-by: Alexander Graf <agraf@suse.de>

hw/ppc/ppc.c

@@ -620,6 +620,13 @@ static void cpu_ppc_tb_start (CPUPPCState *env)
     }
 }
 
+bool ppc_decr_clear_on_delivery(CPUPPCState *env)
+{
+    ppc_tb_t *tb_env = env->tb_env;
+    int flags = PPC_DECR_UNDERFLOW_TRIGGERED | PPC_DECR_UNDERFLOW_LEVEL;
+    return ((tb_env->flags & flags) == PPC_DECR_UNDERFLOW_TRIGGERED);
+}
+
 static inline uint32_t _cpu_ppc_load_decr(CPUPPCState *env, uint64_t next)
 {
     ppc_tb_t *tb_env = env->tb_env;
@@ -677,6 +684,11 @@ static inline void cpu_ppc_decr_excp(PowerPCCPU *cpu)
     ppc_set_irq(cpu, PPC_INTERRUPT_DECR, 1);
 }
 
+static inline void cpu_ppc_decr_lower(PowerPCCPU *cpu)
+{
+    ppc_set_irq(cpu, PPC_INTERRUPT_DECR, 0);
+}
+
 static inline void cpu_ppc_hdecr_excp(PowerPCCPU *cpu)
 {
     /* Raise it */
@@ -684,11 +696,16 @@ static inline void cpu_ppc_hdecr_excp(PowerPCCPU *cpu)
     ppc_set_irq(cpu, PPC_INTERRUPT_HDECR, 1);
 }
 
+static inline void cpu_ppc_hdecr_lower(PowerPCCPU *cpu)
+{
+    ppc_set_irq(cpu, PPC_INTERRUPT_HDECR, 0);
+}
+
 static void __cpu_ppc_store_decr(PowerPCCPU *cpu, uint64_t *nextp,
                                  QEMUTimer *timer,
-                                 void (*raise_excp)(PowerPCCPU *),
+                                 void (*raise_excp)(void *),
-                                 uint32_t decr, uint32_t value,
+                                 void (*lower_excp)(PowerPCCPU *),
-                                 int is_excp)
+                                 uint32_t decr, uint32_t value)
 {
     CPUPPCState *env = &cpu->env;
     ppc_tb_t *tb_env = env->tb_env;
@@ -702,59 +719,74 @@ static void __cpu_ppc_store_decr(PowerPCCPU *cpu, uint64_t *nextp,
         return;
     }
 
+    /*
+     * Going from 2 -> 1, 1 -> 0 or 0 -> -1 is the event to generate a DEC
+     * interrupt.
+     *
+     * If we get a really small DEC value, we can assume that by the time we
+     * handled it we should inject an interrupt already.
+     *
+     * On MSB level based DEC implementations the MSB always means the interrupt
+     * is pending, so raise it on those.
+     *
+     * On MSB edge based DEC implementations the MSB going from 0 -> 1 triggers
+     * an edge interrupt, so raise it here too.
+     */
+    if ((value < 3) ||
+        ((tb_env->flags & PPC_DECR_UNDERFLOW_LEVEL) && (value & 0x80000000)) ||
+        ((tb_env->flags & PPC_DECR_UNDERFLOW_TRIGGERED) && (value & 0x80000000)
+          && !(decr & 0x80000000))) {
+        (*raise_excp)(cpu);
+        return;
+    }
+
+    /* On MSB level based systems a 0 for the MSB stops interrupt delivery */
+    if (!(value & 0x80000000) && (tb_env->flags & PPC_DECR_UNDERFLOW_LEVEL)) {
+        (*lower_excp)(cpu);
+    }
+
+    /* Calculate the next timer event */
     now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
     next = now + muldiv64(value, get_ticks_per_sec(), tb_env->decr_freq);
-    if (is_excp) {
-        next += *nextp - now;
-    }
-    if (next == now) {
-        next++;
-    }
     *nextp = next;
+
     /* Adjust timer */
     timer_mod(timer, next);
-
-    /* If we set a negative value and the decrementer was positive, raise an
-     * exception.
-     */
-    if ((tb_env->flags & PPC_DECR_UNDERFLOW_TRIGGERED)
-        && (value & 0x80000000)
-        && !(decr & 0x80000000)) {
-        (*raise_excp)(cpu);
-    }
 }
 
 static inline void _cpu_ppc_store_decr(PowerPCCPU *cpu, uint32_t decr,
-                                       uint32_t value, int is_excp)
+                                       uint32_t value)
 {
     ppc_tb_t *tb_env = cpu->env.tb_env;
 
     __cpu_ppc_store_decr(cpu, &tb_env->decr_next, tb_env->decr_timer,
-                         &cpu_ppc_decr_excp, decr, value, is_excp);
+                         tb_env->decr_timer->cb, &cpu_ppc_decr_lower, decr,
+                         value);
 }
 
 void cpu_ppc_store_decr (CPUPPCState *env, uint32_t value)
 {
     PowerPCCPU *cpu = ppc_env_get_cpu(env);
 
-    _cpu_ppc_store_decr(cpu, cpu_ppc_load_decr(env), value, 0);
+    _cpu_ppc_store_decr(cpu, cpu_ppc_load_decr(env), value);
 }
 
 static void cpu_ppc_decr_cb(void *opaque)
 {
     PowerPCCPU *cpu = opaque;
 
-    _cpu_ppc_store_decr(cpu, 0x00000000, 0xFFFFFFFF, 1);
+    cpu_ppc_decr_excp(cpu);
 }
 
 static inline void _cpu_ppc_store_hdecr(PowerPCCPU *cpu, uint32_t hdecr,
-                                        uint32_t value, int is_excp)
+                                        uint32_t value)
 {
     ppc_tb_t *tb_env = cpu->env.tb_env;
 
     if (tb_env->hdecr_timer != NULL) {
         __cpu_ppc_store_decr(cpu, &tb_env->hdecr_next, tb_env->hdecr_timer,
-                             &cpu_ppc_hdecr_excp, hdecr, value, is_excp);
+                             tb_env->hdecr_timer->cb, &cpu_ppc_hdecr_lower,
+                             hdecr, value);
     }
 }
 
@@ -762,14 +794,14 @@ void cpu_ppc_store_hdecr (CPUPPCState *env, uint32_t value)
 {
     PowerPCCPU *cpu = ppc_env_get_cpu(env);
 
-    _cpu_ppc_store_hdecr(cpu, cpu_ppc_load_hdecr(env), value, 0);
+    _cpu_ppc_store_hdecr(cpu, cpu_ppc_load_hdecr(env), value);
 }
 
 static void cpu_ppc_hdecr_cb(void *opaque)
 {
     PowerPCCPU *cpu = opaque;
 
-    _cpu_ppc_store_hdecr(cpu, 0x00000000, 0xFFFFFFFF, 1);
+    cpu_ppc_hdecr_excp(cpu);
 }
 
 static void cpu_ppc_store_purr(PowerPCCPU *cpu, uint64_t value)
@@ -792,8 +824,8 @@ static void cpu_ppc_set_tb_clk (void *opaque, uint32_t freq)
      * if a decrementer exception is pending when it enables msr_ee at startup,
      * it's not ready to handle it...
      */
-    _cpu_ppc_store_decr(cpu, 0xFFFFFFFF, 0xFFFFFFFF, 0);
+    _cpu_ppc_store_decr(cpu, 0xFFFFFFFF, 0xFFFFFFFF);
-    _cpu_ppc_store_hdecr(cpu, 0xFFFFFFFF, 0xFFFFFFFF, 0);
+    _cpu_ppc_store_hdecr(cpu, 0xFFFFFFFF, 0xFFFFFFFF);
     cpu_ppc_store_purr(cpu, 0x0000000000000000ULL);
 }
 
@@ -806,6 +838,10 @@ clk_setup_cb cpu_ppc_tb_init (CPUPPCState *env, uint32_t freq)
     tb_env = g_malloc0(sizeof(ppc_tb_t));
     env->tb_env = tb_env;
     tb_env->flags = PPC_DECR_UNDERFLOW_TRIGGERED;
+    if (env->insns_flags & PPC_SEGMENT_64B) {
+        /* All Book3S 64bit CPUs implement level based DEC logic */
+        tb_env->flags |= PPC_DECR_UNDERFLOW_LEVEL;
+    }
     /* Create new timer */
     tb_env->decr_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL, &cpu_ppc_decr_cb, cpu);
     if (0) {

include/hw/ppc/ppc.h

@@ -44,6 +44,9 @@ struct ppc_tb_t {
 #define PPC_DECR_ZERO_TRIGGERED      (1 << 3) /* Decr interrupt triggered when
                                                * the decrementer reaches zero.
                                                */
+#define PPC_DECR_UNDERFLOW_LEVEL     (1 << 4) /* Decr interrupt active when
+                                               * the most significant bit is 1.
+                                               */
 
 uint64_t cpu_ppc_get_tb(ppc_tb_t *tb_env, uint64_t vmclk, int64_t tb_offset);
 clk_setup_cb cpu_ppc_tb_init (CPUPPCState *env, uint32_t freq);

target-ppc/cpu.h

@@ -1133,6 +1133,7 @@ uint64_t cpu_ppc_load_atbl (CPUPPCState *env);
 uint32_t cpu_ppc_load_atbu (CPUPPCState *env);
 void cpu_ppc_store_atbl (CPUPPCState *env, uint32_t value);
 void cpu_ppc_store_atbu (CPUPPCState *env, uint32_t value);
+bool ppc_decr_clear_on_delivery(CPUPPCState *env);
 uint32_t cpu_ppc_load_decr (CPUPPCState *env);
 void cpu_ppc_store_decr (CPUPPCState *env, uint32_t value);
 uint32_t cpu_ppc_load_hdecr (CPUPPCState *env);

target-ppc/excp_helper.c

@@ -723,7 +723,6 @@ void ppc_hw_interrupt(CPUPPCState *env)
     if ((msr_ee != 0 || msr_hv == 0 || msr_pr != 0) && hdice != 0) {
         /* Hypervisor decrementer exception */
         if (env->pending_interrupts & (1 << PPC_INTERRUPT_HDECR)) {
-            env->pending_interrupts &= ~(1 << PPC_INTERRUPT_HDECR);
             powerpc_excp(cpu, env->excp_model, POWERPC_EXCP_HDECR);
             return;
         }
@@ -767,7 +766,9 @@ void ppc_hw_interrupt(CPUPPCState *env)
         }
         /* Decrementer exception */
         if (env->pending_interrupts & (1 << PPC_INTERRUPT_DECR)) {
-            env->pending_interrupts &= ~(1 << PPC_INTERRUPT_DECR);
+            if (ppc_decr_clear_on_delivery(env)) {
+                env->pending_interrupts &= ~(1 << PPC_INTERRUPT_DECR);
+            }
             powerpc_excp(cpu, env->excp_model, POWERPC_EXCP_DECR);
             return;
         }