Commit 33a3ebdc authored Jan 15, 2015 by Oleg Nesterov Committed by Thomas Gleixner Jan 20, 2015

x86, fpu: Don't abuse has_fpu in __kernel_fpu_begin/end()



Now that we have in_kernel_fpu we can remove __thread_clear_has_fpu()
in __kernel_fpu_begin(). And this allows to replace the asymmetrical
and nontrivial use_eager_fpu + tsk_used_math check in kernel_fpu_end()
with the same __thread_has_fpu() check.

The logic becomes really simple; if _begin() does save() then _end()
needs restore(), this is controlled by __thread_has_fpu(). Otherwise
they do clts/stts unless use_eager_fpu().

Not only this makes begin/end symmetrical and imo more understandable,
potentially this allows to change irq_fpu_usable() to avoid all other
checks except "in_kernel_fpu".

Also, with this patch __kernel_fpu_end() does restore_fpu_checking()
and WARNs if it fails instead of math_state_restore(). I think this
looks better because we no longer need __thread_fpu_begin(), and it
would be better to report the failure in this case.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: matt.fleming@intel.com
Cc: bp@suse.de
Cc: pbonzini@redhat.com
Cc: luto@amacapital.net
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Suresh Siddha <sbsiddha@gmail.com>
Link: http://lkml.kernel.org/r/20150115192005.GC27332@redhat.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

parent 14e153ef

arch/x86/kernel/i387.c

+6 −13

Original line number	Diff line number	Diff line
		@@ -81,9 +81,7 @@ void __kernel_fpu_begin(void)
		this_cpu_write(in_kernel_fpu, true);

		if (__thread_has_fpu(me)) {
		__thread_clear_has_fpu(me);
		__save_init_fpu(me);
		/* We do 'stts()' in __kernel_fpu_end() */
		} else if (!use_eager_fpu()) {
		this_cpu_write(fpu_owner_task, NULL);
		clts();
		@@ -93,17 +91,12 @@ EXPORT_SYMBOL(__kernel_fpu_begin);

		void __kernel_fpu_end(void)
		{
		if (use_eager_fpu()) {
		/*
		* For eager fpu, most the time, tsk_used_math() is true.
		* Restore the user math as we are done with the kernel usage.
		* At few instances during thread exit, signal handling etc,
		* tsk_used_math() is false. Those few places will take proper
		* actions, so we don't need to restore the math here.
		*/
		if (likely(tsk_used_math(current)))
		math_state_restore();
		} else {
		struct task_struct *me = current;

		if (__thread_has_fpu(me)) {
		if (WARN_ON(restore_fpu_checking(me)))
		drop_init_fpu(me);
		} else if (!use_eager_fpu()) {
		stts();
		}

Admin message