Building with the attached random configuration file,

  LD      init/built-in.o
arch/x86/built-in.o: In function `pci_xen_swiotlb_init_late':
(.text+0x6b36): undefined reference to `pci_request_acs'
arch/x86/built-in.o: In function `pci_xen_swiotlb_init':
(.init.text+0x2698): undefined reference to `pci_request_acs'
make: *** [vmlinux] Error 1