[PATCH] armv7m_nvic: set DHCSR.DEBUGEN when debugger is attached

[Valentin Ghita]

The DEBUGEN bit is set by the debugger when it is connected to the
core.  Software can use this bit to check if a debug session is active.

Add a function in gdbstub to check if the debugger is attached to a CPU
and use this information when the DHCSR register is read in armv7m_nvic.

Signed-off-by: Valentin Ghita <valentinghita@google.com>
---
 gdbstub.c              | 10 ++++++++++
 hw/intc/armv7m_nvic.c  |  4 ++++
 include/exec/gdbstub.h |  6 ++++++
 3 files changed, 20 insertions(+)

diff --git a/gdbstub.c b/gdbstub.c
index 3c14c6a038..d4e39db8e7 100644
--- a/gdbstub.c
+++ b/gdbstub.c
@@ -3585,6 +3585,16 @@ int gdbserver_start(const char *device)
     return 0;
 }
 
+bool gdb_attached(CPUState *cpu)
+{
+    GDBProcess *process = gdb_get_cpu_process(cpu);
+    if (process != NULL) {
+        return process->attached;
+    }
+
+    return false;
+}
+
 static void register_types(void)
 {
     type_register_static(&char_gdb_type_info);
diff --git a/hw/intc/armv7m_nvic.c b/hw/intc/armv7m_nvic.c
index 13df002ce4..d6fff94bca 100644
--- a/hw/intc/armv7m_nvic.c
+++ b/hw/intc/armv7m_nvic.c
@@ -21,6 +21,7 @@
 #include "sysemu/runstate.h"
 #include "target/arm/cpu.h"
 #include "exec/exec-all.h"
+#include "exec/gdbstub.h"
 #include "exec/memop.h"
 #include "qemu/log.h"
 #include "qemu/module.h"
@@ -1510,6 +1511,9 @@ static uint32_t nvic_readl(NVICState *s, uint32_t offset, 
MemTxAttrs attrs)
         }
         /* We provide minimal-RAS only: RFSR is RAZ/WI */
         return 0;
+    case 0xdf0: /* DHCSR */
+        /* Bit 0: DEBUGEN. */
+        return gdb_attached(CPU(cpu)) ? 1 : 0;
     case 0xf34: /* FPCCR */
         if (!cpu_isar_feature(aa32_vfp_simd, cpu)) {
             return 0;
diff --git a/include/exec/gdbstub.h b/include/exec/gdbstub.h
index a024a0350d..383f4e5224 100644
--- a/include/exec/gdbstub.h
+++ b/include/exec/gdbstub.h
@@ -177,6 +177,12 @@ static inline uint8_t * gdb_get_reg_ptr(GByteArray *buf, 
int len)
  */
 int gdbserver_start(const char *port_or_device);
 
+/**
+ * gdb_attached: check if GDB is attached to a given CPU.
+ * @cpu: the CPU to check if GDB is attached to.
+ */
+bool gdb_attached(CPUState *cpu);
+
 /**
  * gdb_has_xml:
  * This is an ugly hack to cope with both new and old gdb.
-- 
2.35.0.263.gb82422642f-goog