x86/fpu/xstate: Re-enable XSAVES

We did not handle XSAVES instructions correctly. There were issues in
converting between standard and compacted format when interfacing with
user-space. These issues have been corrected.

Add a WARN_ONCE() to make it clear that XSAVES supervisor states are not
yet implemented.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Reviewed-by: Dave Hansen <dave.hansen@intel.com>
Cc: H. Peter Anvin <h.peter.anvin@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi V Shankar <ravi.v.shankar@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1468253937-40008-5-git-send-email-fenghua.yu@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/fpu/init.c

@@ -230,21 +230,6 @@ static void __init fpu__init_system_xstate_size_legacy(void)
 	}
 
 	fpu_user_xstate_size = fpu_kernel_xstate_size;
-
-	/*
-	 * 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);
 }
 
 /*

arch/x86/kernel/fpu/xstate.c

@@ -221,6 +221,15 @@ void fpu__init_cpu_xstate(void)
 {
 	if (!boot_cpu_has(X86_FEATURE_XSAVE) || !xfeatures_mask)
 		return;
+	/*
+	 * Make it clear that XSAVES supervisor states are not yet
+	 * implemented should anyone expect it to work by changing
+	 * bits in XFEATURE_MASK_* macros and XCR0.
+	 */
+	WARN_ONCE((xfeatures_mask & XFEATURE_MASK_SUPERVISOR),
+		"x86/fpu: XSAVES supervisor states are not yet implemented.\n");
+
+	xfeatures_mask &= ~XFEATURE_MASK_SUPERVISOR;
 
 	cr4_set_bits(X86_CR4_OSXSAVE);
 	xsetbv(XCR_XFEATURE_ENABLED_MASK, xfeatures_mask);