As for the management, will some checks when adding mcfg be enough for
management? Or I can maintain a variable to indicate how many space
have been consumed and warn the user if they exceed the threshold?
The latter allows us to do the check when the pxb-pcie is initializing.
I think qemu should not apply any restrictions here.
But will that confuse users when their device is not listed in guest os
while qemu does not throw any error/warning?
Well, that can happen anyway. For example when using an old seabios
version without pci domain support, or other firmware without pci domain
support (coreboot, ovmf). And there is no easy way for qemu to figure
this beforehand.
You can detect this later, when generating the acpi tables, that there
are expander bridges where the hidden pci bar wasn't configured by the
firmware. Logging a warning in that case - pointing out the missing
firmware support - is probably a good idea.