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    SubjectAdd support for RD/WR FS/GSBASE
    Date
    This adds kernel support for some Intel instructions that
    allow fast access to the FS and GS 64bit base. They need
    some changes to entry_64.S because they allow user
    to fully control the GS base.

    Advantages:
    - NMIs (and other "paranoid" interrupts) avoid
    one RDMSR which makes them faster
    - User space can use these instructions, mainly
    for efficient context switching with user thread libraries
    - Context switches do not need to use MSR writes
    anymore to context switch FS/GS base >4GB. This
    will speed up applications that have enough thread
    local data that it won't fit below 4GB.
    - User space can use GS efficiently as an additional
    global pointer register

    I also included one minor (unrelated) optimization to
    disable an unneeded old SWAPGS workaround.

    -Andi



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