x86: filter CPU features dependent on unavailable CPUID levels

Impact: Fixes potential crashes on misconfigured systems.

Some CPU features require specific CPUID levels to be available in
order to function, as they contain information about the operation of
a specific feature.  However, some BIOSes and virtualization software
provide the ability to mask CPUID levels in order to support legacy
operating systems.  We try to enable such CPUID levels when we know
how to do it, but for the remaining cases, filter out such CPU
features when there is no way for us to support them.

Do this in one place, in the CPUID code, with a table-driven approach.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/kernel/cpu/common.c

@@ -212,6 +212,49 @@ static inline void squash_the_stupid_serial_number(struct cpuinfo_x86 *c)
 }
 #endif
 
+/*
+ * Some CPU features depend on higher CPUID levels, which may not always
+ * be available due to CPUID level capping or broken virtualization
+ * software.  Add those features to this table to auto-disable them.
+ */
+struct cpuid_dependent_feature {
+	u32 feature;
+	u32 level;
+};
+static const struct cpuid_dependent_feature __cpuinitconst
+cpuid_dependent_features[] = {
+	{ X86_FEATURE_MWAIT,		0x00000005 },
+	{ X86_FEATURE_DCA,		0x00000009 },
+	{ X86_FEATURE_XSAVE,		0x0000000d },
+	{ 0, 0 }
+};
+
+static void __cpuinit filter_cpuid_features(struct cpuinfo_x86 *c, bool warn)
+{
+	const struct cpuid_dependent_feature *df;
+	for (df = cpuid_dependent_features; df->feature; df++) {
+		/*
+		 * Note: cpuid_level is set to -1 if unavailable, but
+		 * extended_extended_level is set to 0 if unavailable
+		 * and the legitimate extended levels are all negative
+		 * when signed; hence the weird messing around with
+		 * signs here...
+		 */
+		if (cpu_has(c, df->feature) &&
+		    ((s32)df->feature < 0 ?
+		     (u32)df->feature > (u32)c->extended_cpuid_level :
+		     (s32)df->feature > (s32)c->cpuid_level)) {
+			clear_cpu_cap(c, df->feature);
+			if (warn)
+				printk(KERN_WARNING
+				       "CPU: CPU feature %s disabled "
+				       "due to lack of CPUID level 0x%x\n",
+				       x86_cap_flags[df->feature],
+				       df->level);
+		}
+	}
+}	
+
 /*
  * Naming convention should be: <Name> [(<Codename>)]
  * This table only is used unless init_<vendor>() below doesn't set it;
@@ -573,6 +616,7 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
 #ifdef CONFIG_SMP
 	c->cpu_index = boot_cpu_id;
 #endif
+	filter_cpuid_features(c, false);
 }
 
 void __init early_cpu_init(void)
@@ -706,6 +750,9 @@ static void __cpuinit identify_cpu(struct cpuinfo_x86 *c)
 	 * we do "generic changes."
 	 */
 
+	/* Filter out anything that depends on CPUID levels we don't have */
+	filter_cpuid_features(c, true);
+
 	/* If the model name is still unset, do table lookup. */
 	if (!c->x86_model_id[0]) {
 		char *p;