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    SubjectRe: [RFC PATCH 16/22 -v2] add get_monotonic_cycles
    * Paul Mackerras (paulus@samba.org) wrote:
    > Mathieu Desnoyers writes:
    >
    > > Sorry for self-reply, but I thought, in the past, of a way to make this
    > > possible.
    > >
    > > It would imply the creation of a new vsyscall : vgetschedperiod
    > >
    > > It would read a counter that would increment each time the thread is
    > > scheduled out (or in). It would be a per thread counter
    >
    > It's very hard to do a per-thread counter in the VDSO, since threads
    > in the same process see the same memory, by definition. You'd have to
    > have an array of counters and have some way for each thread to know
    > which entry to read. Also you'd have to find space for tens or
    > hundreds of thousands of counters, since there can be that many
    > threads in a process sometimes.
    >
    > Paul.
    >

    Crazy ideas :

    Could we do something along the lines of the thread local storage ?

    Or could we map a per-thread page that would contradict this
    "definition" ?

    Or can we move down the beginning of the user-space thread stack of 4
    bytes (it's already put at a random address anyway) and use these 32
    bits to put our variable ? We don't care if userspace also modifies it;
    the kernel would blindly increment it, so there would be no security
    concerns involved.

    Mathieu

    --
    Mathieu Desnoyers
    Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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