[PATCH 4/4] sim/mips: Recognise a software interrupt request

[Sebastian Huber <sebastian.huber@embedded-brains.de>]

Add support for the Cause.IP0 and Cause.IP1 software generated
interrupts defined by the MIPS Architecture For Programmers Volume III:
The MIPS Privileged Resource Architecture.

Deliver it from the event queue rather than in place, because
signal_exception() leaves the handler address in the program counter for
an interrupt and the instruction being executed would overwrite it.

Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>
---
 sim/mips/interp.c   | 64 +++++++++++++++++++++++++++++++++++++++++++--
 sim/mips/sim-main.h |  2 ++
 2 files changed, 64 insertions(+), 2 deletions(-)

diff --git a/sim/mips/interp.c b/sim/mips/interp.c
index 5dbd1482b99..fddb96d00ed 100644
--- a/sim/mips/interp.c
+++ b/sim/mips/interp.c
@@ -295,6 +295,58 @@ static const OPTION mips_options[] =
 
 int interrupt_pending;
 
+/* An interrupt is requested while the interrupts are enabled and a pending
+   bit of the Cause register meets its mask bit in the Status register:
+
+     Status.IE = 1, Status.EXL = 0, Status.ERL = 0, Cause.IP & Status.IM != 0
+
+   MIPS Architecture For Programmers Volume III: The MIPS Privileged Resource
+   Architecture, the Interrupts chapter.  The R3000 generation, which the
+   R3900 belongs to, has no exception level and disables the interrupts by
+   shifting the interrupt enable stack of its Status register instead, so only
+   the current enable takes part; see the IDT R30xx Family Software Reference
+   Manual, the Status register of the CPU control chapter.  */
+static int
+interrupt_requested (sim_cpu *cpu)
+{
+  if ((SR & status_IE) == 0)
+    return 0;
+
+#ifndef SUBTARGET_R3900
+  if ((SR & (status_EXL | status_ERL)) != 0)
+    return 0;
+#endif
+
+  /* Only the software interrupts.  A hardware interrupt keeps its pending bit
+     set until its device is served. The device model delivers it.  */
+  return ((CAUSE >> cause_IPSW_shift) & (SR >> status_IM_shift)
+	  & cause_IPSW_mask) != 0;
+}
+
+static void
+software_interrupt_event (SIM_DESC sd, void *data)
+{
+  sim_cpu *cpu = STATE_CPU (sd, 0);
+  address_word cia = CPU_PC_GET (cpu);
+
+  /* Recheck, because the write which scheduled this may have been undone in
+     the meantime.  */
+  if (interrupt_requested (cpu))
+    SignalExceptionInterrupt (0);
+}
+
+/* Deliver a pending interrupt at the next instruction boundary.  It cannot be
+   delivered here: signal_exception() leaves the handler address in the program
+   counter for an interrupt and the instruction which is being executed would
+   overwrite it.  This is why the hardware interrupts arrive through the event
+   queue as well.  */
+static void
+check_interrupts (SIM_DESC sd, sim_cpu *cpu)
+{
+  if (interrupt_requested (cpu))
+    sim_events_schedule (sd, 1, software_interrupt_event, NULL);
+}
+
 void
 interrupt_event (SIM_DESC sd, void *data)
 {
@@ -2269,14 +2321,20 @@ decode_coproc (SIM_DESC sd,
 		if (op == cp0_mfc0 || op == cp0_dmfc0)
 		  GPR[rt] = SR;
 		else
-		  SR = GPR[rt];
+		  {
+		    SR = GPR[rt];
+		    check_interrupts (sd, cpu);
+		  }
 		break;
 		/* 13 = Cause              R4000   VR4100  VR4300 */
 	      case 13:
 		if (op == cp0_mfc0 || op == cp0_dmfc0)
 		  GPR[rt] = CAUSE;
 		else
-		  CAUSE = GPR[rt];
+		  {
+		    CAUSE = GPR[rt];
+		    check_interrupts (sd, cpu);
+		  }
 		break;
 		/* 14 = EPC                R4000   VR4100  VR4300 */
 	      case 14:
@@ -2391,6 +2449,7 @@ decode_coproc (SIM_DESC sd,
 	      {
 		PC = EPC;
 		SR &= ~status_EXL;
+		check_interrupts (sd, cpu);
 	      }
 	  }
         else if (op == cp0_rfe && sel == 0x10)
@@ -2401,6 +2460,7 @@ decode_coproc (SIM_DESC sd,
 
 	    /* shift IE/KU history bits right */
 	    SR = LSMASKED32(SR, 31, 4) | LSINSERTED32(LSEXTRACTED32(SR, 5, 2), 3, 0);
+	    check_interrupts (sd, cpu);
 
 	    /* TODO: CACHE register */
 #endif /* SUBTARGET_R3900 */
diff --git a/sim/mips/sim-main.h b/sim/mips/sim-main.h
index b6cb4e12258..3af02e49c0e 100644
--- a/sim/mips/sim-main.h
+++ b/sim/mips/sim-main.h
@@ -557,6 +557,8 @@ struct mips_sim_state {
 #define cause_SW1       (1 << 9)        /* Software interrupt 1 */
 #define cause_IP_mask   (0x3f)          /* Interrupt pending field */
 #define cause_IP_shift  (10)
+#define cause_IPSW_mask (0x3)           /* Software interrupt pending, IP1:IP0 */
+#define cause_IPSW_shift (8)
 
 #define cause_set_EXC(x)  CAUSE = (CAUSE & ~cause_EXC_mask)  | ((x << cause_EXC_shift)  & cause_EXC_mask)
 #define cause_set_EXC2(x) CAUSE = (CAUSE & ~cause_EXC2_mask) | ((x << cause_EXC2_shift) & cause_EXC2_mask)
-- 
2.51.0