PCI: Allow numa_node override via sysfs

NUMA systems with ACPI normally describe the physical topology via _PXM
methods.  But many BIOSes don't implement _PXM, which leaves the kernel
with no way to discover the device topology, which reduces performance
because we can't put memory and processes close to the device.

The NUMA node of a PCI device is already exported in the sysfs "numa_node"
file.  Make that file writable so users can workaround the lack of _PXM
methods in the BIOS.  For example:

  echo 3 > /sys/devices/pci0000:ff/0000:03:1f.3/numa_node

sets the node for PCI device 0000:03:1f.3.

Writing the file emits a FW_BUG warning to encourage users to request
firmware updates.  It also taints the kernel with TAINT_FIRMWARE_WORKAROUND
because overriding the node incorrectly can cause performance issues.

[bhelgaas: changelog, documentation text]
Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Myron Stowe <mstowe@redhat.com>
CC: Alexander Ducyk <alexander.h.duyck@redhat.com>
CC: Jiang Liu <jiang.liu@linux.intel.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-bus-pci b/Documentation/ABI/testing/sysfs-bus-pci
index ee6c04036492..b3bc50f650ee 100644
--- a/Documentation/ABI/testing/sysfs-bus-pci
+++ b/Documentation/ABI/testing/sysfs-bus-pci
@@ -281,3 +281,16 @@ Description:
                 opt-out of driver binding using a driver_override name such as
                 "none".  Only a single driver may be specified in the override,
                 there is no support for parsing delimiters.
+
+What:           /sys/bus/pci/devices/.../numa_node
+Date:           Oct 2014
+Contact:        Prarit Bhargava <prarit@redhat.com>
+Description:
+                This file contains the NUMA node to which the PCI device is
+                attached, or -1 if the node is unknown.  The initial value
+                comes from an ACPI _PXM method or a similar firmware
+                source.  If that is missing or incorrect, this file can be
+                written to override the node.  In that case, please report
+                a firmware bug to the system vendor.  Writing to this file
+                taints the kernel with TAINT_FIRMWARE_WORKAROUND, which
+                reduces the supportability of your system.