[PATCH] hw/timer/hpet: fix IRQ routing in legacy support mode

[David Woodhouse <dwmw2@infradead.org>]

From: David Woodhouse <dwmw@amazon.co.uk>

The interrupt from timer 0 in legacy mode is supposed to go to IRQ 0 on
the i8259 and IRQ 2 on the I/O APIC. The generic x86 GSI handling can't
cope with IRQ numbers differing between the two chips (despite it also
being the case for PCI INTx routing), so add a special case for the HPET.

IRQ 2 isn't valid on the i8259; it's the cascade IRQ and would be
interpreted as spurious interrupt on the secondary PIC. So we can fix
up all attempts to deliver IRQ2, to actually deliver to IRQ0 on the PIC.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 hw/i386/x86.c   | 19 +++++++++++++++----
 hw/timer/hpet.c |  5 ++++-
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/hw/i386/x86.c b/hw/i386/x86.c
index b3d054889b..cb1a8901ff 100644
--- a/hw/i386/x86.c
+++ b/hw/i386/x86.c
@@ -597,13 +597,24 @@ DeviceState *cpu_get_current_apic(void)
 void gsi_handler(void *opaque, int n, int level)
 {
     GSIState *s = opaque;
-
+    int i8259_pin = n;
     trace_x86_gsi_interrupt(n, level);
     switch (n) {
-    case 0 ... ISA_NUM_IRQS - 1:
-        if (s->i8259_irq[n]) {
+    case 2:
+        /*
+         * Special case for HPET legacy mode, which is defined as routing HPET
+         * timer 0 to IRQ2 of the I/O APIC and IRQ0 of the i8259 PIC. Since
+         * IRQ2 on the i8259 is the cascade, it isn't otherwise valid so we
+         * handle it via this special case.
+         */
+        i8259_pin = 0;
+        /* fall through */
+    case 0:
+    case 1:
+    case 3 ... ISA_NUM_IRQS - 1:
+        if (s->i8259_irq[i8259_pin]) {
             /* Under KVM, Kernel will forward to both PIC and IOAPIC */
-            qemu_set_irq(s->i8259_irq[n], level);
+            qemu_set_irq(s->i8259_irq[i8259_pin], level);
         }
         /* fall through */
     case ISA_NUM_IRQS ... IOAPIC_NUM_PINS - 1:
diff --git a/hw/timer/hpet.c b/hw/timer/hpet.c
index 6998094233..9f740ffdee 100644
--- a/hw/timer/hpet.c
+++ b/hw/timer/hpet.c
@@ -196,8 +196,11 @@ static void update_irq(struct HPETTimer *timer, int set)
         /* if LegacyReplacementRoute bit is set, HPET specification requires
          * timer0 be routed to IRQ0 in NON-APIC or IRQ2 in the I/O APIC,
          * timer1 be routed to IRQ8 in NON-APIC or IRQ8 in the I/O APIC.
+         *
+         * There is a special case in the x86 gsi_handler() which converts
+         * IRQ2 into IRQ0 for the i8259 PIC and makes this work correctly.
          */
-        route = (timer->tn == 0) ? 0 : RTC_ISA_IRQ;
+        route = (timer->tn == 0) ? 2 : RTC_ISA_IRQ;
     } else {
         route = timer_int_route(timer);
     }
-- 
2.40.1