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    SubjectAccelerating user mode linux
    Date
    Proposal for a sigaltmm()

    There is a problem with performance when running virtualised environments
    (notably user mode linux). The performance of the mprotect calls needed to
    handle syscalls and protect the UML kernel from its user space are large and
    the alternatives like a seperate process and ptrace are not pretty either

    The cunning plan goes like this

    Add
    current->alt_mm
    A per task flag for 'supervisory' mode


    Tasks start with current->alt_mm NULL and the flag set to supervisory
    On exec/exit tear down alt_mm as well as mm

    Signal delivery checks if alt_mm != NULL && supervisory is clear
    if so it sets supervisory and switches mm/alt_mm, flush the tlb and
    continue handling the signal in the new space

    We add
    sys_switchmm(address);

    This switches to the altmm (creating one if it doesnt exist as a copy of
    the current mm), flushes the tlb and jumps to the address given.

    Any opinions, spanners to throw in the works ?

    Alan
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