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    SubjectRe: [PATCH v2 3/3] i387: support lazy restore of FPU state
    On Mon, 2012-02-20 at 11:48 -0800, 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.

    Reviewing this code, I think we need to set the 'last_cpu' to an invalid
    number in the fpu_alloc too. Appended is the patch.

    From: Suresh Siddha <>
    Subject: x86, fpu: set the last_cpu in fpu_alloc() to an invalid cpu

    Initialize the struct fpu's last_cpu in fpu_alloc() to an invalid cpu number,
    so that the check in fpu_lazy_restore() will always return false
    on the first context-switch in of this new task.

    Otherwise, on a fork(), last_cpu of the new task's fpu will be copied from
    the parent task and fpu_lazy_restore() can potentially return success wrongly
    on the first context-switch in of this new task, leading to fpu corruption.

    Signed-off-by: Suresh Siddha <>
    arch/x86/include/asm/i387.h | 1 +
    1 files changed, 1 insertions(+), 0 deletions(-)

    diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
    index 2479049..58ba656 100644
    --- a/arch/x86/include/asm/i387.h
    +++ b/arch/x86/include/asm/i387.h
    @@ -627,6 +627,7 @@ static inline int fpu_alloc(struct fpu *fpu)
    if (!fpu->state)
    return -ENOMEM;
    WARN_ON((unsigned long)fpu->state & 15);
    + fpu->last_cpu = ~0;
    return 0;

     \ /
      Last update: 2012-02-21 22:57    [W:0.046 / U:8.136 seconds]
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