x86/fpu: Handle #NM without FPU emulation as an error

Don't use CR0.TS.  Make it an error rather than making nonsensical
changes to the FPU state.

(The cond_local_irq_enable() appears to have been pointless, too.)

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kvm list <kvm@vger.kernel.org>
Link: http://lkml.kernel.org/r/f1ee6bf73ed1025fccaab321ba43d0594245f927.1477951965.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/traps.c

@@ -853,6 +853,8 @@ do_spurious_interrupt_bug(struct pt_regs *regs, long error_code)
 dotraplinkage void
 do_device_not_available(struct pt_regs *regs, long error_code)
 {
+	unsigned long cr0;
+
 	RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
 
 #ifdef CONFIG_MATH_EMULATION
@@ -866,10 +868,20 @@ do_device_not_available(struct pt_regs *regs, long error_code)
 		return;
 	}
 #endif
-	fpu__restore(&current->thread.fpu); /* interrupts still off */
-#ifdef CONFIG_X86_32
-	cond_local_irq_enable(regs);
-#endif
+
+	/* This should not happen. */
+	cr0 = read_cr0();
+	if (WARN(cr0 & X86_CR0_TS, "CR0.TS was set")) {
+		/* Try to fix it up and carry on. */
+		write_cr0(cr0 & ~X86_CR0_TS);
+	} else {
+		/*
+		 * Something terrible happened, and we're better off trying
+		 * to kill the task than getting stuck in a never-ending
+		 * loop of #NM faults.
+		 */
+		die("unexpected #NM exception", regs, error_code);
+	}
 }
 NOKPROBE_SYMBOL(do_device_not_available);