migration: Mark CPU states dirty before incoming migration/loadvm

As a rule, CPU internal state should never be updated when
!cpu->kvm_vcpu_dirty (or the HAX equivalent).  If that is done, then
subsequent calls to cpu_synchronize_state() - usually safe and idempotent -
will clobber state.

However, we routinely do this during a loadvm or incoming migration.
Usually this is called shortly after a reset, which will clear all the cpu
dirty flags with cpu_synchronize_all_post_reset().  Nothing is expected
to set the dirty flags again before the cpu state is loaded from the
incoming stream.

This means that it isn't safe to call cpu_synchronize_state() from a
post_load handler, which is non-obvious and potentially inconvenient.

We could cpu_synchronize_all_state() before the loadvm, but that would be
overkill since a) we expect the state to already be synchronized from the
reset and b) we expect to completely rewrite the state with a call to
cpu_synchronize_all_post_init() at the end of qemu_loadvm_state().

To clear this up, this patch introduces cpu_synchronize_pre_loadvm() and
associated helpers, which simply marks the cpu state as dirty without
actually changing anything.  i.e. it says we want to discard any existing
KVM (or HAX) state and replace it with what we're going to load.

Cc: Juan Quintela <quintela@redhat.com>
Cc: Dave Gilbert <dgilbert@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Juan Quintela <quintela@redhat.com>

cpus.c

@@ -921,6 +921,15 @@ void cpu_synchronize_all_post_init(void)
     }
 }
 
+void cpu_synchronize_all_pre_loadvm(void)
+{
+    CPUState *cpu;
+
+    CPU_FOREACH(cpu) {
+        cpu_synchronize_pre_loadvm(cpu);
+    }
+}
+
 static int do_vm_stop(RunState state)
 {
     int ret = 0;

include/sysemu/cpus.h

@@ -27,6 +27,7 @@ void qemu_timer_notify_cb(void *opaque, QEMUClockType type);
 void cpu_synchronize_all_states(void);
 void cpu_synchronize_all_post_reset(void);
 void cpu_synchronize_all_post_init(void);
+void cpu_synchronize_all_pre_loadvm(void);
 
 void qtest_clock_warp(int64_t dest);
 

include/sysemu/hax.h

@@ -33,6 +33,7 @@ int hax_populate_ram(uint64_t va, uint32_t size);
 void hax_cpu_synchronize_state(CPUState *cpu);
 void hax_cpu_synchronize_post_reset(CPUState *cpu);
 void hax_cpu_synchronize_post_init(CPUState *cpu);
+void hax_cpu_synchronize_pre_loadvm(CPUState *cpu);
 
 #ifdef CONFIG_HAX
 

include/sysemu/hw_accel.h

@@ -45,4 +45,14 @@ static inline void cpu_synchronize_post_init(CPUState *cpu)
     }
 }
 
+static inline void cpu_synchronize_pre_loadvm(CPUState *cpu)
+{
+    if (kvm_enabled()) {
+        kvm_cpu_synchronize_pre_loadvm(cpu);
+    }
+    if (hax_enabled()) {
+        hax_cpu_synchronize_pre_loadvm(cpu);
+    }
+}
+
 #endif /* QEMU_HW_ACCEL_H */

include/sysemu/kvm.h

@@ -459,6 +459,7 @@ int kvm_physical_memory_addr_from_host(KVMState *s, void *ram_addr,
 void kvm_cpu_synchronize_state(CPUState *cpu);
 void kvm_cpu_synchronize_post_reset(CPUState *cpu);
 void kvm_cpu_synchronize_post_init(CPUState *cpu);
+void kvm_cpu_synchronize_pre_loadvm(CPUState *cpu);
 
 void kvm_init_cpu_signals(CPUState *cpu);
 

kvm-all.c

@@ -1896,6 +1896,16 @@ void kvm_cpu_synchronize_post_init(CPUState *cpu)
     run_on_cpu(cpu, do_kvm_cpu_synchronize_post_init, RUN_ON_CPU_NULL);
 }
 
+static void do_kvm_cpu_synchronize_pre_loadvm(CPUState *cpu, run_on_cpu_data arg)
+{
+    cpu->kvm_vcpu_dirty = true;
+}
+
+void kvm_cpu_synchronize_pre_loadvm(CPUState *cpu)
+{
+    run_on_cpu(cpu, do_kvm_cpu_synchronize_pre_loadvm, RUN_ON_CPU_NULL);
+}
+
 #ifdef KVM_HAVE_MCE_INJECTION
 static __thread void *pending_sigbus_addr;
 static __thread int pending_sigbus_code;

migration/savevm.c

@@ -1999,6 +1999,8 @@ int qemu_loadvm_state(QEMUFile *f)
         }
     }
 
+    cpu_synchronize_all_pre_loadvm();
+
     ret = qemu_loadvm_state_main(f, mis);
     qemu_event_set(&mis->main_thread_load_event);
 

target/i386/hax-all.c

@@ -635,6 +635,16 @@ void hax_cpu_synchronize_post_init(CPUState *cpu)
     run_on_cpu(cpu, do_hax_cpu_synchronize_post_init, RUN_ON_CPU_NULL);
 }
 
+static void do_hax_cpu_synchronize_pre_loadvm(CPUState *cpu, run_on_cpu_data arg)
+{
+    cpu->hax_vcpu_dirty = true;
+}
+
+void hax_cpu_synchronize_pre_loadvm(CPUState *cpu)
+{
+    run_on_cpu(cpu, do_hax_cpu_synchronize_pre_loadvm, RUN_ON_CPU_NULL);
+}
+
 int hax_smp_cpu_exec(CPUState *cpu)
 {
     CPUArchState *env = (CPUArchState *) (cpu->env_ptr);