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    SubjectA simpler variant on sys_indirect?
    I was just thinking, while sys_indirect is an interesting way to add
    features to a system call, the argument marshalling in user space is a
    bit of a pain.

    An alternate idea would be to instead have a "prefix system call" that
    sets some flags that apply to the next system call made by that thread
    only. They wouldn't be global mode flags that would mess up libraries.

    Maybe I've just been programming x86s too long, but this seems
    like a nicer mental model.

    The downsides are that you need to save and restore the prefix flags
    across signal delivery, and you have a second user/kernel/user transition.

    Most of the options seem to be applied to system calls that resolve
    path names. While that is certainly a very important code path, it's
    also of non-trivial length, even with the dcache. How much would one
    extra kernel entry bloat the budget?

    And if the kernel entry overhead IS a problem, wouldn't you want to
    batch together the non-prefix system calls as well, using something like
    the syslet ideas that were kicked around recently? That would
    allow less than 1 kernel entry per system call, even with prefixes.

    Oh! That suggests an interesting possibility that solves the signal
    handling problem as well:
    - Make a separate prefix system call, BUT
    - The flags are reset on each return to user space, THUS
    - You *have* to use a batch-system-call mechanism for the prefix
    system calls to do anything.

    Of course, this takes us right back to the beginning with respect to
    messy user-space argument marshalling. But at least it's only one
    indirect system call mechanism, not two. Wrapping indirect system call
    mechanism #1 (to set syscall options) in indirect system call mechanism
    #2 (to batch system calls) seems like a bit of a nightmare.

    I'm not at all sure that these are good ideas, but they're not obviously
    bad ones, to me. Is it worth looking for synergy between various
    "indirect system call" ideas?
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      Last update: 2007-06-30 22:55    [W:0.019 / U:1.100 seconds]
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