The vast majority of platforms are not supplying ACPI _PXM (proximity)
information corresponding to host bridge (PNP0A03/PNP0A08) devices
resulting in sysfs "numa_node" values of -1 (NUMA_NO_NODE):

  # for i in /sys/devices/pci0000\:00/*/numa_node; do cat $i; done | uniq
  -1

  # find /sys/ -name "numa_node" | while read fname; do cat $fname; \
    done | uniq
  -1

AMD based platforms provide a fall-back for this situation via amd_bus.c.
These platforms snoop out the information by directly reading specific
registers from the Northbridge and caching them via alloc_pci_root_info().

Later during boot processing when host bridges are discovered -
pci_acpi_scan_root() - the kernel looks for their corresponding ACPI _PXM
method - drivers/acpi/numa.c::acpi_get_node().  If the BIOS supplied a _PXM
method then that node (proximity) value is associated.  If the BIOS did not
supply a _PXM method *and* the platform is AMD-based, the fall-back cached
values obtained directly from the Northbridge are used; otherwise,
"NUMA_NO_NODE" is associated.

There are a number of issues with this fall-back mechanism the most notable
being that amd_bus.c extracts a 3-bit number from a CPU register and uses
it as the node number.  The node numbers used by Linux are logical and
there's no reason they need to be identical to settings in the CPU
registers.  So if we have some node information obtained in the normal way
(from _PXM, SLIT, SRAT, etc.) and some from amd_bus.c, there's no reason to
believe they will be compatible.

This patch warns when this situation occurs:

  pci_root PNP0A08:00: [Firmware Bug]: no _PXM; falling back to node 0 from hardware (may be inconsistent with ACPI node numbers)

Link: https://bugzilla.kernel.org/show_bug.cgi?id=72051Signed-off-by: Myron Stowe <myron.stowe@redhat.com>
Signed-off-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>