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    SubjectRe: Intel P6 vs P7 system call performance
    On Mon, 16 Dec 2002, Linus Torvalds wrote:
    >
    > Ok, I did the vsyscall page too, and tried to make it do the right thing
    > (but I didn't bother to test it on a non-SEP machine).
    >
    > I'm pushing the changes out right now, but basically it boils down to the
    > fact that with these changes, user space can instead of doing an
    >
    > int $0x80
    >
    > instruction for a system call just do a
    >
    > call 0xfffff000

    I thought that last page was intentionally left invalid?

    So that, for example, *(char *)MAP_FAILED will give SIGSEGV;
    whereas now I can read a 0 there (and perhaps you should be
    using get_zeroed_page rather than __get_free_page?).

    I cannot name anything which relies on that page being invalid,
    but think it would be safer to keep that it way; though I guess
    more compatibility pain to use the next page down (or could
    seg lim be used? I forget the granularity restrictions).

    Hugh

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