s390_pci_mmio_write(2) / s390_pci_mmio_read(2)

Purpose

s390_pci_mmio_write()/s390_pci_mmio_read() transfer length bytes between a userspace buffer and a PCI MMIO mapping on System z: plain loads/stores are not sufficient for s390 PCI MMIO areas, so the kernel performs the access with the special instructions/handling the platform requires (s390_pci_mmio_write.2, DESCRIPTION). The address must belong to a PCI MMIO page mapped in the caller’s address space, the transfer must not cross a page boundary, and length cannot exceed the page size (s390_pci_mmio_write.2, DESCRIPTION).

Synopsis

#include <sys/syscall.h>
#include <unistd.h>
int syscall(SYS_s390_pci_mmio_write, unsigned long mmio_addr,
            const void user_buffer[length], size_t length);
int syscall(SYS_s390_pci_mmio_read, unsigned long mmio_addr,
            void user_buffer[length], size_t length);

Arguments and behavior

mmio_addr must land inside a PCI MMIO mapping of the caller; user_buffer/length bound the copy (max one page, no page crossing) (s390_pci_mmio_write.2, DESCRIPTION). This is the read/write pairing used by s390 userspace drivers (the vfio-style map + syscall access pattern); generic x86-style mmap’d MMIO access just dereferences, which is why the calls are s390-only.

Return value and errors

0 on success; -1 with errno (s390_pci_mmio_write.2, RETURN VALUE). Errors: EFAULT (invalid mmio_addr, or user_buffer outside the caller’s address space), EINVAL (invalid length), ENODEV (PCI support not enabled), ENOMEM (insufficient kernel memory) (s390_pci_mmio_write.2, ERRORS).

Flags and constants

None.

Security and notes

The permission story is the mapping, not the syscall: the caller must already have mmap’d the device region (with the usual device-file permissions), and the syscall only executes the platform-correct access. Interview angle: why would a platform need a syscall to read memory it can map? – because s390 PCI MMIO accesses require special instructions/error handling that plain userspace loads do not provide. Cross-reference: mmap, ioperm, pciconfig_read.

References

• https://man7.org/linux/man-pages/man2/s390_pci_mmio_write.2.html
• https://man7.org/linux/man-pages/man2/s390_pci_mmio_read.2.html
• man-pages syscalls(2) (3.19, s390 only)