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>

1 files changed, 2 insertions, 0 deletions

diff --git a/migration/savevm.c b/migration/savevm.c
index 035c127fdd..1993ca23fe 100644
--- a/migration/savevm.c
+++ b/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);