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    SubjectRe: [PATCH v2 3/3] i387: support lazy restore of FPU state
    On Mon, Feb 20, 2012 at 2:48 PM, Linus Torvalds
    <> wrote:
    > From: Linus Torvalds <>
    > Date: Sun, 19 Feb 2012 13:27:00 -0800
    > Subject: [PATCH v2 3/3] i387: support lazy restore of FPU state
    > This makes us recognize when we try to restore FPU state that matches
    > what we already have in the FPU on this CPU, and avoids the restore
    > entirely if so.
    > To do this, we add two new data fields:
    >  - a percpu 'fpu_owner_task' variable that gets written any time we
    >   update the "has_fpu" field, and thus acts as a kind of back-pointer
    >   to the task that owns the CPU.  The exception is when we save the FPU
    >   state as part of a context switch - if the save can keep the FPU
    >   state around, we leave the 'fpu_owner_task' variable pointing at the
    >   task whose FP state still remains on the CPU.
    >  - a per-thread 'last_cpu' field, that indicates which CPU that thread
    >   used its FPU on last.  We update this on every context switch
    >   (writing an invalid CPU number if the last context switch didn't
    >   leave the FPU in a lazily usable state), so we know that *that*
    >   thread has done nothing else with the FPU since.
    > These two fields together can be used when next switching back to the
    > task to see if the CPU still matches: if 'fpu_owner_task' matches the
    > task we are switching to, we know that no other task (or kernel FPU
    > usage) touched the FPU on this CPU in the meantime, and if the current
    > CPU number matches the 'last_cpu' field, we know that this thread did no
    > other FP work on any other CPU, so the FPU state on the CPU must match
    > what was saved on last context switch.
    > In that case, we can avoid the 'f[x]rstor' entirely, and just clear the
    > CR0.TS bit.
    > Signed-off-by: Linus Torvalds <>

    I haven't tried really figuring this out yet, but building the Fedora kernel
    on x86_64 with your latest tree results in:

    ERROR: "fpu_owner_task" [lib/raid6/raid6_pq.ko] undefined!
    ERROR: "fpu_owner_task" [arch/x86/kvm/kvm.ko] undefined!
    ERROR: "fpu_owner_task" [arch/x86/crypto/sha1-ssse3.ko] undefined!
    ERROR: "fpu_owner_task" [arch/x86/crypto/serpent-sse2-x86_64.ko] undefined!
    ERROR: "fpu_owner_task" [arch/x86/crypto/ghash-clmulni-intel.ko] undefined!
    make[1]: *** [__modpost] Error 1
    make: *** [modules] Error 2
    + exit 1

    Since this patch went in as 7e16838d94b566a1, I'm guessing it's at least

    I'm building again with more verbose output but I thought I'd send this out

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      Last update: 2012-02-21 02:53    [W:0.023 / U:5.600 seconds]
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