Question: why no VMEXIT occur when I read/write emulated pci devices's bar by mmap inside guest userspace?

[Wu Zongyong]

Hi,

I create a VM with a virtual pci device "-net nic,model=e1000", and I
attemped to read/write the bar0 of the virtual nic inside the guest
userspace by mmap the "/sys/bus/pci/devices/xxxx:xx:xx.x/resource0" to 
the userspace.
What I expected is a VMEXIT should occur when I read/write the bar but I
didn't see any VMEXIT event.
So I'm confused with this and any detailed analysis about mmio handle
routine would be appreciated.

Thanks