Is that OK if I use the following code?

...

	if (is_in_pci_mem_space(addr)) {
		if (!user_mode(regs)) {
			ret =3D probe_kernel_address(regs->nip, inst);
=20
			if (!ret) {
				rd =3D get_rt(inst);
				regs->gpr[rd] =3D 0xffffffff;
			}
		}

Don't you need to check that the instruction is actually
a memory read?

I also know that there are people mapping PCIe addresses
directly into userspace for 'simple' access to things like
fpga devices.

I suspect that such devices are the ones likely to generate
the faulting cycles. So you probably do want to handle
faults from normal userspace addresses.

	David