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    SubjectRe: Intel P6 vs P7 system call performance
    Linus Torvalds wrote:

    > Yeah, it's not very convenient. I didn't find any real alternatives,
    > though, and you can always just put 0xfffff000 in memory or registers and
    > jump to that.

    Putting the value into memory myself is not possible. In a DSO I have
    to address memory indirectly. But all registers (except %ebp, and maybe
    it'll be used some day) are used at the time of the call.

    But there is a way: if I'm using

    #define makesyscall(name) \
    movl $__NR_##name, $eax; \
    call 0xfffff000-__NR_##name($eax)

    and you'd put at address 0xfffff000 the address of the entry point the
    wrappers wouldn't have any problems finding it.


    > In fact, I suspect that if you actually want to use it in
    > glibc, then at least in the short term that's what you need to do anyway,
    > sinc eyou probably don't want to have a glibc that only works with very
    > recent kernels.

    That's a compilation option. We might want to do dynamic testing and
    yes, a simple pointer indirection is adequate.

    But still, the problem is detecting the capable kernels. You have said
    not long ago that comparing kernel versions is wrong. And I agree. It
    doesn't cover backports and nothing. But there is a lack of an alternative.

    If you don't like the process-global page thingy (anymore) the
    alternative would be a sysconf() system call.

    --
    --------------. ,-. 444 Castro Street
    Ulrich Drepper \ ,-----------------' \ Mountain View, CA 94041 USA
    Red Hat `--' drepper at redhat.com `---------------------------

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