x86: Allow FPU to be used at interrupt time even with eagerfpu

x86: Allow FPU to be used at interrupt time even with eagerfpu

With the addition of eagerfpu the irq_fpu_usable() now returns false
negatives especially in the case of ksoftirqd and interrupted idle task,
two common cases for FPU use for example in networking/crypto.  With
eagerfpu=off FPU use is possible in those contexts.  This is because of
the eagerfpu check in interrupted_kernel_fpu_idle():

...
  * For now, with eagerfpu we will return interrupted kernel FPU
  * state as not-idle. TBD: Ideally we can change the return value
  * to something like __thread_has_fpu(current). But we need to
  * be careful of doing __thread_clear_has_fpu() before saving
  * the FPU etc for supporting nested uses etc. For now, take
  * the simple route!
...
 	if (use_eager_fpu())
 		return 0;

As eagerfpu is automatically "on" on those CPUs that also have the
features like AES-NI this patch changes the eagerfpu check to return 1 in
case the kernel_fpu_begin() has not been said yet.  Once it has been the
__thread_has_fpu() will start returning 0.

Notice that with eagerfpu the __thread_has_fpu is always true initially.
FPU use is thus always possible no matter what task is under us, unless
the state has already been saved with kernel_fpu_begin().

[ hpa: this is a performance regression, not a correctness regression,
  but since it can be quite serious on CPUs which need encryption at
  interrupt time I am marking this for urgent/stable. ]

Signed-off-by: Pekka Riikonen <priikone@iki.fi>
Link: http://lkml.kernel.org/r/alpine.GSO.2.00.1305131356320.18@git.silcnet.org
Cc: <stable@vger.kernel.org> v3.7+
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>