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    SubjectRe: Intel P6 vs P7 system call performance


    On Tue, 17 Dec 2002, Ulrich Drepper wrote:
    >
    > The problem with the current solution is the instruction set of the x86.
    > In your test code you simply use call 0xfffff000 and it magically work.
    > But this is only the case because your program is linked statically.

    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. 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.

    So I was actually assuming that the glibc code would look more like
    something like this:

    old_fashioned:
    int $0x80
    ret

    unsigned long system_call_ptr = old_fashioned;

    /* .. startup .. */
    if (kernel_version > xxx)
    system_call_ptr = 0xfffff000;


    /* ... usage ... */
    call *system_call_ptr;

    since you cannot depend on the 0xfffff000 on older kernels anyway..

    > Instead I've changed the syscall handling to effectve do
    >
    > pushl %ebp
    > movl $0xfffff000, %ebp
    > call *%ebp
    > popl %ebp

    The above will work, but then you'd have limited yourself to a binary that
    _only_ works on new kernels. So I'd suggest the memory indirection
    instead.

    Linus

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