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    Subject[PATCH 13/24] x86/fpu: Only write PKRU if it is different from current
    Date
    Dave Hansen says that the `wrpkru' is more expensive than `rdpkru'. It
    has a higher cycle cost and it's also practically a (light) speculation
    barrier.

    As an optimisation read the current PKRU value and only write the new
    one if it is different.

    Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    ---
    arch/x86/include/asm/special_insns.h | 6 ++++++
    1 file changed, 6 insertions(+)

    diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
    index 27328606ff687..28ffdf0c1add4 100644
    --- a/arch/x86/include/asm/special_insns.h
    +++ b/arch/x86/include/asm/special_insns.h
    @@ -121,6 +121,12 @@ static inline void __write_pkru_ins(u32 pkru)

    static inline void __write_pkru(u32 pkru)
    {
    + /*
    + * WRPKRU is relatively expensive compared to RDPKRU.
    + * Avoid WRPKRU when it would not change the value.
    + */
    + if (pkru == __read_pkru_ins())
    + return;
    __write_pkru_ins(pkru);
    }

    --
    2.20.1
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