x86/fpu: Simplify and speed up fpu__copy()

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: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Eric Biggers <ebiggers3@gmail.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yu-cheng Yu <yu-cheng.yu@intel.com>
Link: http://lkml.kernel.org/r/20170923130016.21448-32-mingo@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 3 insertions, 12 deletions

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 77668d91fdc1..52122dd418ae 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);