Re: [RFC] speeding up the stat() family of system calls...

["H. Peter Anvin" <hpa@zytor.com>]

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

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