Commit e10078eb authored Sep 23, 2017 by Ingo Molnar

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>

parent 7f1487c5

arch/x86/kernel/fpu/core.c

+3 −12

Original line number	Diff line number	Diff line
		@@ -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);

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