Without further ado, the following was found:

Issue:    to the user-space buffer I<user_buffer> → and writes them to the user-space buffer I<user_buffer>

"The B<s390_pci_mmio_write>()  system call writes I<length> bytes of data "
"from the user-space buffer I<user_buffer> to the PCI MMIO memory location "
"specified by I<mmio_addr>.  The B<s390_pci_mmio_read>()  system call reads "
"I<length> bytes of data from the PCI MMIO memory location specified by "
"I<mmio_addr> to the user-space buffer I<user_buffer>."