x86/fpu: Fix CPUID-less FPU detection

The old code didn't work at all because it adjusted the current caps
instead of the forced caps.  Anything it did would be undone later
during CPU identification.  Fix that and, while we're at it, improve
the logging and don't bother running it if CPUID is available.

Reported-by: Matthew Whitehead <[email protected]>
Signed-off-by: Andy Lutomirski <[email protected]>
Reviewed-by: Borislav Petkov <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Fenghua Yu <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: One Thousand Gnomes <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Yu-cheng Yu <[email protected]>
Link: http://lkml.kernel.org/r/f1134e30cafa73c4e2e68119e9741793622cfd15.1484705016.git.luto@kernel.org
Signed-off-by: Ingo Molnar <[email protected]>

Author: amluto

arch/x86/kernel/fpu/init.c

@@ -48,13 +48,7 @@ void fpu__init_cpu(void)
 	fpu__init_cpu_xstate();
 }
 
-/*
- * The earliest FPU detection code.
- *
- * Set the X86_FEATURE_FPU CPU-capability bit based on
- * trying to execute an actual sequence of FPU instructions:
- */
-static void fpu__init_system_early_generic(struct cpuinfo_x86 *c)
+static bool fpu__probe_without_cpuid(void)
 {
 	unsigned long cr0;
 	u16 fsw, fcw;
@@ -65,14 +59,21 @@ static void fpu__init_system_early_generic(struct cpuinfo_x86 *c)
 	cr0 &= ~(X86_CR0_TS | X86_CR0_EM);
 	write_cr0(cr0);
 
-	if (!test_bit(X86_FEATURE_FPU, (unsigned long *)cpu_caps_cleared)) {
-		asm volatile("fninit ; fnstsw %0 ; fnstcw %1"
-			     : "+m" (fsw), "+m" (fcw));
+	asm volatile("fninit ; fnstsw %0 ; fnstcw %1" : "+m" (fsw), "+m" (fcw));
+
+	pr_info("x86/fpu: Probing for FPU: FSW=0x%04hx FCW=0x%04hx\n", fsw, fcw);
 
-		if (fsw == 0 && (fcw & 0x103f) == 0x003f)
-			set_cpu_cap(c, X86_FEATURE_FPU);
+	return fsw == 0 && (fcw & 0x103f) == 0x003f;
+}
+
+static void fpu__init_system_early_generic(struct cpuinfo_x86 *c)
+{
+	if (!boot_cpu_has(X86_FEATURE_CPUID) &&
+	    !test_bit(X86_FEATURE_FPU, (unsigned long *)cpu_caps_cleared)) {
+		if (fpu__probe_without_cpuid())
+			setup_force_cpu_cap(X86_FEATURE_FPU);
 		else
-			clear_cpu_cap(c, X86_FEATURE_FPU);
+			setup_clear_cpu_cap(X86_FEATURE_FPU);
 	}
 
 #ifndef CONFIG_MATH_EMULATION