KVM: Rework VCPU state writeback API

This grand cleanup drops all reset and vmsave/load related
synchronization points in favor of four(!) generic hooks:

- cpu_synchronize_all_states in qemu_savevm_state_complete
  (initial sync from kernel before vmsave)
- cpu_synchronize_all_post_init in qemu_loadvm_state
  (writeback after vmload)
- cpu_synchronize_all_post_init in main after machine init
- cpu_synchronize_all_post_reset in qemu_system_reset
  (writeback after system reset)

These writeback points + the existing one of VCPU exec after
cpu_synchronize_state map on three levels of writeback:

- KVM_PUT_RUNTIME_STATE (during runtime, other VCPUs continue to run)
- KVM_PUT_RESET_STATE   (on synchronous system reset, all VCPUs stopped)
- KVM_PUT_FULL_STATE    (on init or vmload, all VCPUs stopped as well)

This level is passed to the arch-specific VCPU state writing function
that will decide which concrete substates need to be written. That way,
no writer of load, save or reset functions that interact with in-kernel
KVM states will ever have to worry about synchronization again. That
also means that a lot of reasons for races, segfaults and deadlocks are
eliminated.

cpu_synchronize_state remains untouched, just as Anthony suggested. We
continue to need it before reading or writing of VCPU states that are
also tracked by in-kernel KVM subsystems.

Consequently, this patch removes many cpu_synchronize_state calls that
are now redundant, just like remaining explicit register syncs.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

vl.c

@@ -3002,6 +3002,33 @@ static void nographic_update(void *opaque)
     qemu_mod_timer(nographic_timer, interval + qemu_get_clock(rt_clock));
 }
 
+void cpu_synchronize_all_states(void)
+{
+    CPUState *cpu;
+
+    for (cpu = first_cpu; cpu; cpu = cpu->next_cpu) {
+        cpu_synchronize_state(cpu);
+    }
+}
+
+void cpu_synchronize_all_post_reset(void)
+{
+    CPUState *cpu;
+
+    for (cpu = first_cpu; cpu; cpu = cpu->next_cpu) {
+        cpu_synchronize_post_reset(cpu);
+    }
+}
+
+void cpu_synchronize_all_post_init(void)
+{
+    CPUState *cpu;
+
+    for (cpu = first_cpu; cpu; cpu = cpu->next_cpu) {
+        cpu_synchronize_post_init(cpu);
+    }
+}
+
 struct vm_change_state_entry {
     VMChangeStateHandler *cb;
     void *opaque;
@@ -3143,6 +3170,7 @@ void qemu_system_reset(void)
     QTAILQ_FOREACH_SAFE(re, &reset_handlers, entry, nre) {
         re->func(re->opaque);
     }
+    cpu_synchronize_all_post_reset();
 }
 
 void qemu_system_reset_request(void)
@@ -5928,6 +5956,7 @@ int main(int argc, char **argv, char **envp)
     machine->init(ram_size, boot_devices,
                   kernel_filename, kernel_cmdline, initrd_filename, cpu_model);
 
+    cpu_synchronize_all_post_init();
 
 #ifndef _WIN32
     /* must be after terminal init, SDL library changes signal handlers */