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    SubjectRe: [patch 06/11] syslets: core, documentation
    On Wed, 14 Feb 2007, Benjamin LaHaise wrote:

    > On Wed, Feb 14, 2007 at 03:17:59PM -0800, Davide Libenzi wrote:
    > > > That's an incorrect assumption. Every task/thread in the system has FPU
    > > > state associated with it, in part due to the fact that glibc has to change
    > > > some of the rounding mode bits, making them different than the default from
    > > > a freshly initialized state.
    > >
    > > IMO I still belive this is not a huge problem. FPU state propagation/copy
    > > can be done in a clever way, once we detect the in-async condition.
    >
    > Show me. clts() and stts() are expensive hardware operations which there
    > is no means of avoiding as control register writes impact the CPU in a not
    > trivial manner. I've spent far too much time staring at profiles of what
    > goes on in the context switch code in the process of looking for optimizations
    > on this very issue to be ignored on this point.

    The trivial case is the cachehit case. Everything flows like usual since
    we don't swap threads.
    If we're going to sleep, __async_schedule has to save/copy (depending if
    TS_USEDFPU is set) the current FPU state to the newly selected service
    thread (return-to-userspace thread).
    When a fault eventually happen in the new userspace thread, context is
    restored.



    - Davide


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