[VirtIO-Serial] IO hang on Windows


I have a windows server that version is 2008 R2, and get performance data through qemu-guest-agent.
I can not read performance data for some windows VMs on the Hypervisor.
I have no idea why it happened, and can not appear it agent.
I have debug the qemu-ga, find out that qemu-ga try to write data to a virtio-serial device named \.\Global\org.qemu.guest_agent.1 get a error return ERROR_IO_PENDING, and try to write forever.
So, is there any way or tools to test the serial-device is work?
And what should i do to find out the problems on my VM?