x86/fpu: Do CLTS fpu__init_system()

mxcsr_feature_mask_init() depends on TS being cleared, as it executes
an FXSAVE instruction.

After later changes we will move the TS setup into fpu__init_cpu(),
which will interact with this - so clear the TS flag explicitly.
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>

arch/x86/kernel/fpu/init.c

@@ -213,6 +213,13 @@ void fpu__init_system(void)
 	/* The FPU has to be operational for some of the later FPU init activities: */
 	fpu__init_cpu();
 
+	/*
+	 * But don't leave CR0::TS set yet, as some of the FPU setup methods depend
+	 * on being able to execute FPU instructions that will fault on a set TS,
+	 * such as the FXSAVE in mxcsr_feature_mask_init().
+	 */
+	clts();
+
 	/*
 	 * Set up the legacy init FPU context. (xstate init might overwrite this
 	 * with a more modern format, if the CPU supports it.)