x86/fpu: Simplify FPU handling by embedding the fpstate in task_struct (again)

x86/fpu: Simplify FPU handling by embedding the fpstate in task_struct (again)

So 6 years ago we made the FPU fpstate dynamically allocated:

  aa283f49276e ("x86, fpu: lazy allocation of FPU area - v5")
  61c4628b5386 ("x86, fpu: split FPU state from task struct - v5")

In hindsight this was a mistake:

   - it complicated context allocation failure handling, such as:

		/* kthread execs. TODO: cleanup this horror. */
		if (WARN_ON(fpstate_alloc_init(fpu)))
			force_sig(SIGKILL, tsk);

   - it caused us to enable irqs in fpu__restore():

                local_irq_enable();
                /*
                 * does a slab alloc which can sleep
                 */
                if (fpstate_alloc_init(fpu)) {
                        /*
                         * ran out of memory!
                         */
                        do_group_exit(SIGKILL);
                        return;
                }
                local_irq_disable();

   - it (slightly) slowed down task creation/destruction by adding
     slab allocation/free pattens.

   - it made access to context contents (slightly) slower by adding
     one more pointer dereference.

The motivation for the dynamic allocation was two-fold:

   - reduce memory consumption by non-FPU tasks

   - allocate and handle only the necessary amount of context for
     various XSAVE processors that have varying hardware frame
     sizes.

These days, with glibc using SSE memcpy by default and GCC optimizing
for SSE/AVX by default, the scope of FPU using apps on an x86 system is
much larger than it was 6 years ago.

For example on a freshly installed Fedora 21 desktop system, with a
recent kernel, all non-kthread tasks have used the FPU shortly after
bootup.

Also, even modern embedded x86 CPUs try to support the latest vector
instruction set - so they'll too often use the larger xstate frame
sizes.

So remove the dynamic allocation complication by embedding the FPU
fpstate in task_struct again. This should make the FPU a lot more
accessible to all sorts of atomic contexts.

We could still optimize for the xstate frame size in the future,
by moving the state structure to the last element of task_struct,
and allocating only a part of that.

This change is kept minimal by still keeping the ctx_alloc()/free()
routines (that now do nothing substantial) - we'll remove them in
the following patches.

Reviewed-by: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@linux.intel.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>