Without this change, the majority of the raw PCI config space access functions silently ignore a non-zero segment argument, which is certainly wrong. Apart from pci_direct_conf1, all other non-MMCFG access methods get used only for non-extended accesses (i.e. assigned to raw_pci_ops only). Consequently, with the way raw_pci_{read,write}() work, it would be a coding error to call these functions with a non-zero segment (with the current call flow this cannot happen afaict). The access method 1 accessor, as it can be used for extended accesses (on AMD systems) instead gets checks added for the passed in segment to be zero. This would be the case when on such a system having multiple PCI segments (don't know whether any exist in practice) MMCFG for some reason is not usable, and method 1 gets selected for doing extended accesses. Rather than accessing the wrong device's config space, the function will now error out. v2: Convert BUG_ON() to WARN_ON(), and extend description as per Ingo's request. Signed-off-by: Jan Beulich <jbeulich@novell.com> Reviewed-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>