H87のマザボにPCIブリッジがついていて、vt-dを有効にすると、dmesgにこんなん出るんだけど、なんですかね？

[ 0.336527] PCI-DMA: Intel(R) Virtualization Technology for Directed I/O
[ 0.336563] ------------[ cut here ]------------
[ 0.336569] WARNING: at /build/buildd/linux-3.8.0/drivers/pci/search.c:46 pci_find_upstream_pcie_bridge+0x55/0x80()
[ 0.336572] Hardware name: To Be Filled By O.E.M.
[ 0.336573] Modules linked in:
[ 0.336576] Pid: 1, comm: swapper/0 Not tainted 3.8.0-25-generic #37-Ubuntu
[ 0.336578] Call Trace:
[ 0.336582] [<ffffffff810587ef>] warn_slowpath_common+0x7f/0xc0
[ 0.336584] [<ffffffff8105884a>] warn_slowpath_null+0x1a/0x20
[ 0.336587] [<ffffffff8137c345>] pci_find_upstream_pcie_bridge+0x55/0x80
[ 0.336592] [<ffffffff815a25eb>] intel_iommu_add_device+0x4b/0x1f0
[ 0.336594] [<ffffffff81599ff0>] ? bus_set_iommu+0x60/0x60
[ 0.336596] [<ffffffff8159a023>] add_iommu_group+0x33/0x60
[ 0.336600] [<ffffffff8145403d>] bus_for_each_dev+0x5d/0xa0
[ 0.336602] [<ffffffff81599fdb>] bus_set_iommu+0x4b/0x60
[ 0.336606] [<ffffffff81d474b5>] intel_iommu_init+0x34f/0x41d
[ 0.336610] [<ffffffff81d0e3f3>] ? memblock_find_dma_reserve+0x13d/0x13d
[ 0.336613] [<ffffffff81d0e405>] pci_iommu_init+0x12/0x3c
[ 0.336616] [<ffffffff8100215a>] do_one_initcall+0x12a/0x180
[ 0.336619] [<ffffffff81d05d32>] kernel_init_freeable+0x13e/0x1c6
[ 0.336621] [<ffffffff81d05582>] ? loglevel+0x31/0x31
[ 0.336625] [<ffffffff816a9cc0>] ? rest_init+0x80/0x80
[ 0.336627] [<ffffffff816a9cce>] kernel_init+0xe/0xf0
[ 0.336631] [<ffffffff816d3fac>] ret_from_fork+0x7c/0xb0
[ 0.336634] [<ffffffff816a9cc0>] ? rest_init+0x80/0x80
[ 0.336638] ---[ end trace 34a1982ad5ee97a4 ]---