There is a race between TCG and accesses to the dirty log:

      vCPU thread                  reader thread
      -----------------------      -----------------------
      TLB check -> slow path
        notdirty_mem_write
          write to RAM
          set dirty flag
                                   clear dirty flag
      TLB check -> fast path
                                   read memory
        write to RAM

Fortunately, in order to fix it, no change is required to the
vCPU thread.  However, the reader thread must delay the read after
the vCPU thread has finished the write.  This can be approximated
conservatively by run_on_cpu, which waits for the end of the current
translation block.

A similar technique is used by KVM, which has to do a synchronous TLB
flush after doing a test-and-clear of the dirty-page flags.
Reported-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>