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    Subject[PATCH 31/33] x86/fpu: Simplify and speed up fpu__copy()
    Date
    fpu__copy() has a preempt_disable()/enable() pair, which it had to do to
    be able to atomically unlazy the current task when doing an FNSAVE.

    But we don't unlazy tasks anymore, we always do direct saves/restores of
    FPU context.

    So remove both the unnecessary critical section, and update the comments.

    Cc: Andy Lutomirski <luto@amacapital.net>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: Eric Biggers <ebiggers3@gmail.com>
    Cc: Fenghua Yu <fenghua.yu@intel.com>
    Cc: H. Peter Anvin <hpa@zytor.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    arch/x86/kernel/fpu/core.c | 15 +++------------
    1 file changed, 3 insertions(+), 12 deletions(-)

    diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
    index 4acfc0ebc160..e935f8faf287 100644
    --- a/arch/x86/kernel/fpu/core.c
    +++ b/arch/x86/kernel/fpu/core.c
    @@ -206,22 +206,13 @@ int fpu__copy(struct fpu *dst_fpu, struct fpu *src_fpu)
    * Save current FPU registers directly into the child
    * FPU context, without any memory-to-memory copying.
    *
    - * We have to do all this with preemption disabled,
    - * mostly because of the FNSAVE case, because in that
    - * case we must not allow preemption in the window
    - * between the FNSAVE and us marking the context lazy.
    - *
    - * It shouldn't be an issue as even FNSAVE is plenty
    - * fast in terms of critical section length.
    + * ( The function 'fails' in the FNSAVE case, which destroys
    + * register contents so we have to copy them back. )
    */
    - preempt_disable();
    if (!copy_fpregs_to_fpstate(dst_fpu)) {
    - memcpy(&src_fpu->state, &dst_fpu->state,
    - fpu_kernel_xstate_size);
    -
    + memcpy(&src_fpu->state, &dst_fpu->state, fpu_kernel_xstate_size);
    copy_kernel_to_fpregs(&src_fpu->state);
    }
    - preempt_enable();

    trace_x86_fpu_copy_src(src_fpu);
    trace_x86_fpu_copy_dst(dst_fpu);
    --
    2.11.0
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