i8254: Rework & fix interaction with HPET in legacy mode

When the HPET enters legacy mode, the IRQ output of the PIT is
suppressed and replaced by the HPET timer 0. But the current code to
emulate this was broken in many ways. It reset the PIT state after
re-enabling, it worked against a stale static PIT structure, and it did
not properly saved/restored the IRQ output mask in the PIT vmstate.

This patch solves the PIT IRQ control in a different way. On x86, it
both redirects the PIT IRQ to the HPET, just like the RTC. But it also
keeps the control line from the HPET to the PIT. This allows to disable
the PIT QEMU timer when it is not needed. The PIT's view on the control
line state is now saved in the same format that qemu-kvm is already
using.

Note that, in contrast to the suppressed RTC IRQ line, we do not need to
save/restore the PIT line state in the HPET. As we trigger a PIT IRQ
update via the control line, the line state is reconstructed on mode
switch.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/hpet.c

@@ -65,6 +65,7 @@ typedef struct HPETState {
     qemu_irq irqs[HPET_NUM_IRQ_ROUTES];
     uint32_t flags;
     uint8_t rtc_irq_level;
+    qemu_irq pit_enabled;
     uint8_t num_timers;
     HPETTimer timer[HPET_MAX_TIMERS];
 
@@ -573,12 +574,15 @@ static void hpet_ram_write(void *opaque, target_phys_addr_t addr,
                     hpet_del_timer(&s->timer[i]);
                 }
             }
-            /* i8254 and RTC are disabled when HPET is in legacy mode */
+            /* i8254 and RTC output pins are disabled
+             * when HPET is in legacy mode */
             if (activating_bit(old_val, new_val, HPET_CFG_LEGACY)) {
-                hpet_pit_disable();
+                qemu_set_irq(s->pit_enabled, 0);
+                qemu_irq_lower(s->irqs[0]);
                 qemu_irq_lower(s->irqs[RTC_ISA_IRQ]);
             } else if (deactivating_bit(old_val, new_val, HPET_CFG_LEGACY)) {
-                hpet_pit_enable();
+                qemu_irq_lower(s->irqs[0]);
+                qemu_set_irq(s->pit_enabled, 1);
                 qemu_set_irq(s->irqs[RTC_ISA_IRQ], s->rtc_irq_level);
             }
             break;
@@ -632,7 +636,6 @@ static void hpet_reset(DeviceState *d)
 {
     HPETState *s = FROM_SYSBUS(HPETState, sysbus_from_qdev(d));
     int i;
-    static int count = 0;
 
     for (i = 0; i < s->num_timers; i++) {
         HPETTimer *timer = &s->timer[i];
@@ -649,32 +652,30 @@ static void hpet_reset(DeviceState *d)
         timer->wrap_flag = 0;
     }
 
+    qemu_set_irq(s->pit_enabled, 1);
     s->hpet_counter = 0ULL;
     s->hpet_offset = 0ULL;
     s->config = 0ULL;
-    if (count > 0) {
-        /* we don't enable pit when hpet_reset is first called (by hpet_init)
-         * because hpet is taking over for pit here. On subsequent invocations,
-         * hpet_reset is called due to system reset. At this point control must
-         * be returned to pit until SW reenables hpet.
-         */
-        hpet_pit_enable();
-    }
     hpet_cfg.hpet[s->hpet_id].event_timer_block_id = (uint32_t)s->capability;
     hpet_cfg.hpet[s->hpet_id].address = sysbus_from_qdev(d)->mmio[0].addr;
-    count = 1;
 
     /* to document that the RTC lowers its output on reset as well */
     s->rtc_irq_level = 0;
 }
 
-static void hpet_handle_rtc_irq(void *opaque, int n, int level)
+static void hpet_handle_legacy_irq(void *opaque, int n, int level)
 {
     HPETState *s = FROM_SYSBUS(HPETState, opaque);
 
-    s->rtc_irq_level = level;
-    if (!hpet_in_legacy_mode(s)) {
-        qemu_set_irq(s->irqs[RTC_ISA_IRQ], level);
+    if (n == HPET_LEGACY_PIT_INT) {
+        if (!hpet_in_legacy_mode(s)) {
+            qemu_set_irq(s->irqs[0], level);
+        }
+    } else {
+        s->rtc_irq_level = level;
+        if (!hpet_in_legacy_mode(s)) {
+            qemu_set_irq(s->irqs[RTC_ISA_IRQ], level);
+        }
     }
 }
 
@@ -717,7 +718,8 @@ static int hpet_init(SysBusDevice *dev)
     s->capability |= (s->num_timers - 1) << HPET_ID_NUM_TIM_SHIFT;
     s->capability |= ((HPET_CLK_PERIOD) << 32);
 
-    qdev_init_gpio_in(&dev->qdev, hpet_handle_rtc_irq, 1);
+    qdev_init_gpio_in(&dev->qdev, hpet_handle_legacy_irq, 2);
+    qdev_init_gpio_out(&dev->qdev, &s->pit_enabled, 1);
 
     /* HPET Area */
     memory_region_init_io(&s->iomem, &hpet_ram_ops, s, "hpet", 0x400);

hw/hpet_emul.h

@@ -22,6 +22,9 @@
 
 #define HPET_NUM_IRQ_ROUTES     32
 
+#define HPET_LEGACY_PIT_INT     0
+#define HPET_LEGACY_RTC_INT     1
+
 #define HPET_CFG_ENABLE 0x001
 #define HPET_CFG_LEGACY 0x002
 

hw/i8254.c

@@ -52,6 +52,7 @@ typedef struct PITChannelState {
     int64_t next_transition_time;
     QEMUTimer *irq_timer;
     qemu_irq irq;
+    uint32_t irq_disabled;
 } PITChannelState;
 
 typedef struct PITState {
@@ -61,8 +62,6 @@ typedef struct PITState {
     PITChannelState channels[3];
 } PITState;
 
-static PITState pit_state;
-
 static void pit_irq_timer_update(PITChannelState *s, int64_t current_time);
 
 static int pit_get_count(PITChannelState *s)
@@ -378,8 +377,9 @@ static void pit_irq_timer_update(PITChannelState *s, int64_t current_time)
     int64_t expire_time;
     int irq_level;
 
-    if (!s->irq_timer)
+    if (!s->irq_timer || s->irq_disabled) {
         return;
+    }
     expire_time = pit_get_next_transition_time(s, current_time);
     irq_level = pit_get_out1(s, current_time);
     qemu_set_irq(s->irq, irq_level);
@@ -450,6 +450,7 @@ static int pit_load_old(QEMUFile *f, void *opaque, int version_id)
         qemu_get_8s(f, &s->bcd);
         qemu_get_8s(f, &s->gate);
         s->count_load_time=qemu_get_be64(f);
+        s->irq_disabled = 0;
         if (s->irq_timer) {
             s->next_transition_time=qemu_get_be64(f);
             qemu_get_timer(f, s->irq_timer);
@@ -460,11 +461,12 @@ static int pit_load_old(QEMUFile *f, void *opaque, int version_id)
 
 static const VMStateDescription vmstate_pit = {
     .name = "i8254",
-    .version_id = 2,
+    .version_id = 3,
     .minimum_version_id = 2,
     .minimum_version_id_old = 1,
     .load_state_old = pit_load_old,
     .fields      = (VMStateField []) {
+        VMSTATE_UINT32_V(channels[0].irq_disabled, PITState, 3),
         VMSTATE_STRUCT_ARRAY(channels, PITState, 3, 2, vmstate_pit_channel, PITChannelState),
         VMSTATE_TIMER(channels[0].irq_timer, PITState),
         VMSTATE_END_OF_LIST()
@@ -483,7 +485,7 @@ static void pit_reset(DeviceState *dev)
         s->gate = (i != 2);
         s->count_load_time = qemu_get_clock_ns(vm_clock);
         s->count = 0x10000;
-        if (i == 0) {
+        if (i == 0 && !s->irq_disabled) {
             s->next_transition_time =
                 pit_get_next_transition_time(s, s->count_load_time);
             qemu_mod_timer(s->irq_timer, s->next_transition_time);
@@ -491,26 +493,20 @@ static void pit_reset(DeviceState *dev)
     }
 }
 
-/* When HPET is operating in legacy mode, i8254 timer0 is disabled */
-void hpet_pit_disable(void) {
-    PITChannelState *s;
-    s = &pit_state.channels[0];
-    if (s->irq_timer)
-        qemu_del_timer(s->irq_timer);
-}
-
-/* When HPET is reset or leaving legacy mode, it must reenable i8254
- * timer 0
- */
-
-void hpet_pit_enable(void)
+/* When HPET is operating in legacy mode, suppress the ignored timer IRQ,
+ * reenable it when legacy mode is left again. */
+static void pit_irq_control(void *opaque, int n, int enable)
 {
-    PITState *pit = &pit_state;
-    PITChannelState *s;
-    s = &pit->channels[0];
-    s->mode = 3;
-    s->gate = 1;
-    pit_load_count(s, 0);
+    PITState *pit = opaque;
+    PITChannelState *s = &pit->channels[0];
+
+    if (enable) {
+        s->irq_disabled = 0;
+        pit_irq_timer_update(s, qemu_get_clock_ns(vm_clock));
+    } else {
+        s->irq_disabled = 1;
+        qemu_del_timer(s->irq_timer);
+    }
 }
 
 static const MemoryRegionPortio pit_portio[] = {
@@ -536,6 +532,8 @@ static int pit_initfn(ISADevice *dev)
     memory_region_init_io(&pit->ioports, &pit_ioport_ops, pit, "pit", 4);
     isa_register_ioport(dev, &pit->ioports, pit->iobase);
 
+    qdev_init_gpio_in(&dev->qdev, pit_irq_control, 1);
+
     qdev_set_legacy_instance_id(&dev->qdev, pit->iobase, 2);
 
     return 0;

hw/i8254.h

@@ -50,7 +50,4 @@ int pit_get_initial_count(ISADevice *dev, int channel);
 int pit_get_mode(ISADevice *dev, int channel);
 int pit_get_out(ISADevice *dev, int channel, int64_t current_time);
 
-void hpet_pit_disable(void);
-void hpet_pit_enable(void);
-
 #endif /* !HW_I8254_H */

hw/pc.c

@@ -1139,6 +1139,9 @@ void pc_basic_device_init(ISABus *isa_bus, qemu_irq *gsi,
 {
     int i;
     DriveInfo *fd[MAX_FD];
+    DeviceState *hpet = NULL;
+    int pit_isa_irq = 0;
+    qemu_irq pit_alt_irq = NULL;
     qemu_irq rtc_irq = NULL;
     qemu_irq *a20_line;
     ISADevice *i8042, *port92, *vmmouse, *pit;
@@ -1149,20 +1152,26 @@ void pc_basic_device_init(ISABus *isa_bus, qemu_irq *gsi,
     register_ioport_write(0xf0, 1, 1, ioportF0_write, NULL);
 
     if (!no_hpet) {
-        DeviceState *hpet = sysbus_try_create_simple("hpet", HPET_BASE, NULL);
+        hpet = sysbus_try_create_simple("hpet", HPET_BASE, NULL);
 
         if (hpet) {
             for (i = 0; i < GSI_NUM_PINS; i++) {
                 sysbus_connect_irq(sysbus_from_qdev(hpet), i, gsi[i]);
             }
-            rtc_irq = qdev_get_gpio_in(hpet, 0);
+            pit_isa_irq = -1;
+            pit_alt_irq = qdev_get_gpio_in(hpet, HPET_LEGACY_PIT_INT);
+            rtc_irq = qdev_get_gpio_in(hpet, HPET_LEGACY_RTC_INT);
         }
     }
     *rtc_state = rtc_init(isa_bus, 2000, rtc_irq);
 
     qemu_register_boot_set(pc_boot_set, *rtc_state);
 
-    pit = pit_init(isa_bus, 0x40, 0, NULL);
+    pit = pit_init(isa_bus, 0x40, pit_isa_irq, pit_alt_irq);
+    if (hpet) {
+        /* connect PIT to output control line of the HPET */
+        qdev_connect_gpio_out(hpet, 0, qdev_get_gpio_in(&pit->qdev, 0));
+    }
     pcspk_init(pit);
 
     for(i = 0; i < MAX_SERIAL_PORTS; i++) {