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    SubjectRe: Voyager subarchitecture for 2.5.46
    On Sun, Nov 10, 2002 at 10:59:55AM -0800, Linus Torvalds wrote:

    > On Sun, 10 Nov 2002, Pavel Machek wrote:
    > >
    > > Unfortunately, this means "bye bye vsyscalls for gettimeofday".
    >
    > Not necessarily. All of the fastpatch and the checking can be done by the
    > vsyscall, and if the vsyscall notices that there is a backwards jump in
    > time it just gives up and does a real system call. The vsyscall does need
    > to figure out the CPU it's running on somehow, but that should be solvable
    > - indexing through the thread ID or something.

    I'm planning to store the CPU number in the highest bits of the TSC ...

    > That said, I suspect that the real issue with vsyscalls is that they don't
    > really make much sense. The only system call we've ever found that matters
    > at all is gettimeofday(), and the vsyscall implementation there looks like
    > a "cool idea, but doesn't really matter (and complicates things a lot)".

    It's not complicating things overly. We'd have to go through most of the
    hoops anyway if we wanted a fast gettimeofday syscall instead of a
    vsyscall.

    > The system call overhead tends to scale up very well with CPU speed (the
    > one esception being the P4 which just has some internal problems with "int
    > 0x80" and slowed down compared to a PIII).
    >
    > So I would just suggest not spending a lot of effort on it, considering
    > the problems it already has.

    Agreed. The only problem left I see is the need to have an interrupt of
    every CPU from time to time to update the per-cpu time values, and to
    synchronize those to the 'global timer interrupt' somehow.

    --
    Vojtech Pavlik
    SuSE Labs
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