x86/fpu: Disable XSAVES* support for now

The kernel's handling of 'compacted' xsave state layout is buggy:

    http://marc.info/?l=linux-kernel&m=142967852317199

I don't have such a system, and the description there is vague, but
from extrapolation I guess that there were two kinds of bugs
observed:

  - boot crashes, due to size calculations being wrong and the dynamic
    allocation allocating a too small xstate area. (This is now fixed
    in the new FPU code - but still present in stable kernels.)

  - FPU state corruption and ABI breakage: if signal handlers try to
    change the FPU state in standard format, which then the kernel
    tries to restore in the compacted format.

These breakages are scary, but they only occur on a small number of
systems that have XSAVES* CPU support. Yet we have had XSAVES support
in the upstream kernel for a large number of stable kernel releases,
and the fixes are involved and unproven.

So do the safe resolution first: disable XSAVES* support and only
use the standard xstate format. This makes the code work and is
easy to backport.

On top of this we can work on enabling (and testing!) proper
compacted format support, without backporting pressure, on top of the
new, cleaned up FPU code.

Cc: <stable@vger.kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
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>

arch/x86/kernel/i387.c

@@ -173,6 +173,21 @@ static void init_thread_xstate(void)
 		xstate_size = sizeof(struct i387_fxsave_struct);
 	else
 		xstate_size = sizeof(struct i387_fsave_struct);
+
+	/*
+	 * Quirk: we don't yet handle the XSAVES* instructions
+	 * correctly, as we don't correctly convert between
+	 * standard and compacted format when interfacing
+	 * with user-space - so disable it for now.
+	 *
+	 * The difference is small: with recent CPUs the
+	 * compacted format is only marginally smaller than
+	 * the standard FPU state format.
+	 *
+	 * ( This is easy to backport while we are fixing
+	 *   XSAVES* support. )
+	 */
+	setup_clear_cpu_cap(X86_FEATURE_XSAVES);
 }
 
 /*