accel/mshv: Add vCPU signal handling

Implement signal handling for MSHV vCPUs to support asynchronous
interrupts from the main thread.

Signed-off-by: Magnus Kulke <magnuskulke@linux.microsoft.com>
Link: https://lore.kernel.org/r/20250916164847.77883-12-magnuskulke@linux.microsoft.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

accel/mshv/mshv-all.c

@@ -524,6 +524,35 @@ static int mshv_cpu_exec(CPUState *cpu)
     return ret;
 }
 
+/*
+ * The signal handler is triggered when QEMU's main thread receives a SIG_IPI
+ * (SIGUSR1). This signal causes the current CPU thread to be kicked, forcing a
+ * VM exit on the CPU. The VM exit generates an exit reason that breaks the loop
+ * (see mshv_cpu_exec). If the exit is due to a Ctrl+A+x command, the system
+ * will shut down. For other cases, the system will continue running.
+ */
+static void sa_ipi_handler(int sig)
+{
+    /* TODO: call IOCTL to set_immediate_exit, once implemented. */
+
+    qemu_cpu_kick_self();
+}
+
+static void init_signal(CPUState *cpu)
+{
+    /* init cpu signals */
+    struct sigaction sigact;
+    sigset_t set;
+
+    memset(&sigact, 0, sizeof(sigact));
+    sigact.sa_handler = sa_ipi_handler;
+    sigaction(SIG_IPI, &sigact, NULL);
+
+    pthread_sigmask(SIG_BLOCK, NULL, &set);
+    sigdelset(&set, SIG_IPI);
+    pthread_sigmask(SIG_SETMASK, &set, NULL);
+}
+
 static void *mshv_vcpu_thread(void *arg)
 {
     CPUState *cpu = arg;
@@ -540,6 +569,7 @@ static void *mshv_vcpu_thread(void *arg)
         error_report("Failed to init vcpu %d", cpu->cpu_index);
         goto cleanup;
     }
+    init_signal(cpu);
 
     /* signal CPU creation */
     cpu_thread_signal_created(cpu);