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    SubjectRe: [RFC] speeding up the stat() family of system calls...
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    Ok the sign bit doesn't really make any sense on second thought... to work with set_fs() we have to load something from memory anyway and then we might as well do a compare...

    "H. Peter Anvin" <hpa@zytor.com> wrote:
    >On 12/26/2013 11:00 AM, Linus Torvalds wrote:
    >>
    >> Interestingly, looking at the cp_new_stat() profiles, the games we
    >> play to get efficient range checking seem to actually hurt us. Maybe
    >> it's the "sbb" that is just expensive, or maybe it's turning a (very
    >> predictable) conditional branch into a data dependency chain instead.
    >> Or maybe it's just random noise in my profiles that happened to make
    >> those sbb's look bad.
    >>
    >
    >I'm not at all surprised... there is a pretty serious data dependency
    >chain here and in the end we end up manifesting a value in a register
    >that has to be tested even though it is available in the flags. Inline
    >assembly also means the compiler can't optimize it at all.
    >
    >I have to wonder if we actually have to test the upper limit, though:
    >we
    >can always guarantee a guard zone between user space and kernel space,
    >and thus guarantee either a #PF or #GP if someone tries to overflow
    >user
    >space. Testing just the lower limit would be much cheaper, especially
    >on 64 bits where we can simply test the sign bit.
    >
    >What do you think?
    >
    > -hpa

    --
    Sent from my mobile phone. Please pardon brevity and lack of formatting.


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