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    SubjectRe: [PATCH 2/2] i387: support lazy restore of FPU state
    On 02/19/2012 02:44 PM, H. Peter Anvin wrote:
    > On 02/19/2012 02:37 PM, Linus Torvalds wrote:
    >>
    >> - on *every* task switch from task A, we write A->thread.fpu.last_cpu,
    >> whether we owned the FPU or not. And we only write a real CPU number in
    >> the case where we owned it, and the FPU save left the state untouched
    >> in the FPU.
    >>
    >> - so when we switch into task A next time, comparing the current CPU
    >> number with that 'last_cpu' field inarguably says "when I last switched
    >> out, I really saved it on this CPU"
    >>
    >> That, together with verifying that the per-cpu "fpu_owner_task" matches
    >> "task A", guarantees that the state is really valid. Because we will
    >> clear (or set to another task) fpu_owner_task if it ever gets
    >> switched to anything else.
    >>
    >> But somebody should really validate this. Think through all the
    >> kernel_fpu_begin() etc cases. I think it looks pretty obvious, and it
    >> really does seem to work and improve task switching, but...
    >>
    >
    > I think your logic is correct but suboptimal.
    >
    > What would make more sense to me is that we write last_cpu when we
    > *load* the state. After all, if you didn't load the state you couldn't
    > have modified it. In kernel_fpu_begin, *if* we end up flushing the
    > state, we should set last_cpu to -1 indicating that *no* CPU currently
    > owns the state -- after all, even on this CPU we would now have to
    > reload the state from memory.
    >

    This is obviously wrong for kernel_fpu_begin... what we should do there
    is to just set fpu_owner_task to NULL as we no longer have any task's
    content in the fpu; no need to much with last_cpu though.

    -hpa



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