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    Subject[tip:x86/urgent] x86: Fix fake apicid to node mapping for numa emulation
    Commit-ID:  b0c4d952a158a6a2547672cf4fc9d55e415410de
    Author: David Rientjes <>
    AuthorDate: Thu, 6 May 2010 02:24:34 -0700
    Committer: Ingo Molnar <>
    CommitDate: Thu, 6 May 2010 12:02:05 +0200

    x86: Fix fake apicid to node mapping for numa emulation

    With NUMA emulation, it's possible for a single cpu to be bound
    to multiple nodes since more than one may have affinity if
    allocated on a physical node that is local to the cpu.

    APIC ids must therefore be mapped to the lowest node ids to
    maintain generic kernel use of functions such as cpu_to_node()
    that determine device affinity. For example, if a device has
    proximity to physical node 1, for instance, and a cpu happens to
    be mapped to a higher emulated node id 8, the proximity may not
    be correctly determined by comparison in generic code even
    though the cpu may be truly local and allocated on physical node 1.

    When this happens, the true topology of the machine isn't
    accurately represented in the emulated environment; although
    this isn't critical to the system's uptime, any generic code
    that is NUMA aware benefits from the physical topology being
    accurately represented.

    This can affect any system that maps multiple APIC ids to a
    single node and is booted with numa=fake=N where N is greater
    than the number of physical nodes.

    Signed-off-by: David Rientjes <>
    Cc: Yinghai Lu <>
    Cc: Suresh Siddha <>
    LKML-Reference: <>
    Signed-off-by: Ingo Molnar <>
    arch/x86/mm/srat_64.c | 3 ++-
    1 files changed, 2 insertions(+), 1 deletions(-)

    diff --git a/arch/x86/mm/srat_64.c b/arch/x86/mm/srat_64.c
    index 28c6876..38512d0 100644
    --- a/arch/x86/mm/srat_64.c
    +++ b/arch/x86/mm/srat_64.c
    @@ -461,7 +461,8 @@ void __init acpi_fake_nodes(const struct bootnode *fake_nodes, int num_nodes)
    * node, it must now point to the fake node ID.
    for (j = 0; j < MAX_LOCAL_APIC; j++)
    - if (apicid_to_node[j] == nid)
    + if (apicid_to_node[j] == nid &&
    + fake_apicid_to_node[j] == NUMA_NO_NODE)
    fake_apicid_to_node[j] = i;
    for (i = 0; i < num_nodes; i++)

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