As discussed previously in a qemu-devel thread: https://www.mail-archive.com/qemu-devel@xxxxxxxxxx/msg725141.html s390x has the notion of unlinked VFs being available at the LPAR-level (Virtual Functions where the kernel does not have access to the associated Physical Function). These devices are currently not marked as is_virtfn. There seems to be some precedent (ex: in powerpc, eeh_debugfs_break_device()) where pdev->is_virtfn && pdev->physfn == 0 is used to detect these sort of detached VFs. We toyed with the idea of doing this but it causes additional fallout as various other areas of kernel code have an expectation that is_virtfn=1 implies there is a linked PF available to the kernel. In the s390x case, the firmware layer underneath handles the VF emulation as it still has access to the PF that the LPAR (and thus the kernel) cannot see. But one thing this firmware layer does not do is emulate the PCI_COMMAND_MEMORY bit, which was OK until vfio-pci started enforcing it via abafbc55. The vfio-pci check is waived for VFs as of ebfa440c, but vfio-pci can't actually tell that these particular devices are VFs. The proposed patch attempts to identify these detached VFs and subsequently provide this information to vfio-pci so that it knows to also accept the lack of PCI_COMMAND_MEMORY for these sorts of devices. For now the bit is only set for s390x but other architectures could opt in to it as well if needed. Matthew Rosato (1): PCI: Introduce flag for detached virtual functions arch/s390/pci/pci.c | 8 ++++++++ drivers/vfio/pci/vfio_pci_config.c | 3 ++- include/linux/pci.h | 1 + 3 files changed, 11 insertions(+), 1 deletion(-) -- 1.8.3.1
