kvm-qemu: Proper vm_stop on debug events

When a vcpu exits after hitting a debug exception, we have to invoke vm_stop(EXCP_DEBUG). But this has to take place over the io-thread.

This patch introduces kvm_debug_stop_requested to signal this event, and it takes care that the interrupted vcpu itself goes immediately into stop state.

Signed-off-by Jan Kiszka <[email protected]>

---
 qemu/qemu-kvm.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

Index: b/qemu/qemu-kvm.c
===================================================================

qemu/qemu-kvm.c +0 -0

 static inline unsigned long kvm_get_thread_id(void)
 {

     return syscall(SYS_gettid);

qemu/qemu-kvm.c +4 -0

@@ -517,6 +519,10 @@ int kvm_main_loop(void)
             qemu_system_powerdown();
         else if (qemu_reset_requested())
 	    qemu_kvm_system_reset();
+	else if (kvm_debug_stop_requested) {
+	    kvm_debug_stop_requested = 0;
+	    vm_stop(EXCP_DEBUG);
+	}
     }
 
     pause_all_threads();

qemu/qemu-kvm.c +0 -1

     return 1;
 }
 
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Quoted 17 lines · Jan Kiszka

When a vcpu exits after hitting a debug exception, we have to invoke vm_stop(EXCP_DEBUG). But this has to take place over the io-thread.

This patch introduces kvm_debug_stop_requested to signal this event, and it takes care that the interrupted vcpu itself goes immediately into stop state.

Signed-off-by Jan Kiszka <[email protected]>

---
 qemu/qemu-kvm.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

Index: b/qemu/qemu-kvm.c
===================================================================

qemu/qemu-kvm.c pthread_t io_thread; · line 63

Quoted 6 lines

qemu/qemu-kvm.c +2 -0

@@ -58,6 +58,8 @@ pthread_t io_thread;
 static int io_thread_fd = -1;
 static int io_thread_sigfd = -1;
 
+static int kvm_debug_stop_requested;
+
 

Why use this instead of just keying off of exception_index == EXCP_DEBUG?

Quoted 18 lines

qemu/qemu-kvm.c +0 -0

 static inline unsigned long kvm_get_thread_id(void)
 {

     return syscall(SYS_gettid);

qemu/qemu-kvm.c +4 -0

@@ -517,6 +519,10 @@ int kvm_main_loop(void)
             qemu_system_powerdown();
         else if (qemu_reset_requested())
         qemu_kvm_system_reset();
+    else if (kvm_debug_stop_requested) {
+        kvm_debug_stop_requested = 0;
+        vm_stop(EXCP_DEBUG);
+    }
     }
 
     pause_all_threads();

@@ -529,7 +535,8 @@ static int kvm_debug(void *opaque, int v
 {
     CPUState *env = cpu_single_env;
 

qemu/qemu-kvm.c static int kvm_debug(void *opaque, int v · line 542

Quoted 4 lines

qemu/qemu-kvm.c +2 -1

-    env->exception_index = EXCP_DEBUG;
+    kvm_debug_stop_requested = 1;
+    vcpu_info[vcpu].stopped = 1;
 

This isn't quite right. In the very least, you need to set stopping = 0 and signal on the qemu_pause_cond. Thinking it through more though, a breakpoint should stop all VCPUs, right?

vm_stop(EXCP_DEBUG) will actually do this. It invokes the vm_state_notify callbacks and the io-thread registers one. I think you should probably just issue vm_stop(EXCP_DEBUG) from kvm_debug.