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>

1 files changed, 20 insertions, 18 deletions

diff --git a/hw/hpet.c b/hw/hpet.c
index 16833fa..fd3ddca 100644
--- a/hw/hpet.c
+++ b/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);