[PATCH for-5.1? 2/3] hw/intc/armv7m_nvic: Provide default "reset the system" behaviour for SYSRESETREQ

[Peter Maydell <peter.maydell@linaro.org>]

The NVIC provides an outbound qemu_irq "SYSRESETREQ" which it signals
when the guest sets the SYSRESETREQ bit in the AIRCR register.  This
matches the hardware design (where the CPU has a signal of this name
and it is up to the SoC to connect that up to an actual reset
mechanism), but in QEMU it mostly results in duplicated code in SoC
objects and bugs where SoC model implementors forget to wire up the
SYSRESETREQ line.

Provide a default behaviour for the case where SYSRESETREQ is not
actually connected to anything: use qemu_system_reset_request() to
perform a system reset.  This will allow us to remove the
implementations of SYSRESETREQ handling from the boards where that's
exactly what it does, and also fixes the bugs in the board models
which forgot to wire up the signal:

 * microbit
 * mps2-an385
 * mps2-an505
 * mps2-an511
 * mps2-an521
 * musca-a
 * musca-b1
 * netduino
 * netduinoplus2

We still allow the board to wire up the signal if it needs to, in case
we need to model more complicated reset controller logic or to model
buggy SoC hardware which forgot to wire up the line itself. But
defaulting to "reset the system" is more often going to be correct
than defaulting to "do nothing".

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 include/hw/arm/armv7m.h |  4 +++-
 hw/intc/armv7m_nvic.c   | 17 ++++++++++++++++-
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/include/hw/arm/armv7m.h b/include/hw/arm/armv7m.h
index d2c74d3872a..a30e3c64715 100644
--- a/include/hw/arm/armv7m.h
+++ b/include/hw/arm/armv7m.h
@@ -35,7 +35,9 @@ typedef struct {
 
 /* ARMv7M container object.
  * + Unnamed GPIO input lines: external IRQ lines for the NVIC
- * + Named GPIO output SYSRESETREQ: signalled for guest AIRCR.SYSRESETREQ
+ * + Named GPIO output SYSRESETREQ: signalled for guest AIRCR.SYSRESETREQ.
+ *   If this GPIO is not wired up then the NVIC will default to performing
+ *   a qemu_system_reset_request(SHUTDOWN_CAUSE_GUEST_RESET).
  * + Property "cpu-type": CPU type to instantiate
  * + Property "num-irq": number of external IRQ lines
  * + Property "memory": MemoryRegion defining the physical address space
diff --git a/hw/intc/armv7m_nvic.c b/hw/intc/armv7m_nvic.c
index 3c4b6e6d701..277a98b87b9 100644
--- a/hw/intc/armv7m_nvic.c
+++ b/hw/intc/armv7m_nvic.c
@@ -19,6 +19,7 @@
 #include "hw/intc/armv7m_nvic.h"
 #include "hw/irq.h"
 #include "hw/qdev-properties.h"
+#include "sysemu/runstate.h"
 #include "target/arm/cpu.h"
 #include "exec/exec-all.h"
 #include "exec/memop.h"
@@ -64,6 +65,20 @@ static const uint8_t nvic_id[] = {
     0x00, 0xb0, 0x1b, 0x00, 0x0d, 0xe0, 0x05, 0xb1
 };
 
+static void signal_sysresetreq(NVICState *s)
+{
+    if (qemu_irq_is_connected(s->sysresetreq)) {
+        qemu_irq_pulse(s->sysresetreq);
+    } else {
+        /*
+         * Default behaviour if the SoC doesn't need to wire up
+         * SYSRESETREQ (eg to a system reset controller of some kind):
+         * perform a system reset via the usual QEMU API.
+         */
+        qemu_system_reset_request(SHUTDOWN_CAUSE_GUEST_RESET);
+    }
+}
+
 static int nvic_pending_prio(NVICState *s)
 {
     /* return the group priority of the current pending interrupt,
@@ -1524,7 +1539,7 @@ static void nvic_writel(NVICState *s, uint32_t offset, uint32_t value,
             if (value & R_V7M_AIRCR_SYSRESETREQ_MASK) {
                 if (attrs.secure ||
                     !(cpu->env.v7m.aircr & R_V7M_AIRCR_SYSRESETREQS_MASK)) {
-                    qemu_irq_pulse(s->sysresetreq);
+                    signal_sysresetreq(s);
                 }
             }
             if (value & R_V7M_AIRCR_VECTCLRACTIVE_MASK) {
-- 
2.20.1