KVM: x86: Expand build-time assertion on reverse CPUID usage

Add build-time checks to ensure KVM isn't trying to do a reverse CPUID
lookup on Linux-defined feature bits, along with comments to explain
the gory details of X86_FEATUREs and bit().

No functional change intended.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/cpuid.c

@@ -281,8 +281,9 @@ int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
 	return r;
 }
 
-static void cpuid_mask(u32 *word, int wordnum)
+static __always_inline void cpuid_mask(u32 *word, int wordnum)
 {
+	reverse_cpuid_check(wordnum);
 	*word &= boot_cpu_data.x86_capability[wordnum];
 }
 

arch/x86/kvm/cpuid.h

@@ -56,18 +56,41 @@ static const struct cpuid_reg reverse_cpuid[] = {
 	[CPUID_7_1_EAX]       = {         7, 1, CPUID_EAX},
 };
 
-static inline u32 bit(int bitno)
+/*
+ * Reverse CPUID and its derivatives can only be used for hardware-defined
+ * feature words, i.e. words whose bits directly correspond to a CPUID leaf.
+ * Retrieving a feature bit or masking guest CPUID from a Linux-defined word
+ * is nonsensical as the bit number/mask is an arbitrary software-defined value
+ * and can't be used by KVM to query/control guest capabilities.  And obviously
+ * the leaf being queried must have an entry in the lookup table.
+ */
+static __always_inline void reverse_cpuid_check(unsigned x86_leaf)
 {
-	return BIT(bitno & 31);
+	BUILD_BUG_ON(x86_leaf == CPUID_LNX_1);
+	BUILD_BUG_ON(x86_leaf == CPUID_LNX_2);
+	BUILD_BUG_ON(x86_leaf == CPUID_LNX_3);
+	BUILD_BUG_ON(x86_leaf == CPUID_LNX_4);
+	BUILD_BUG_ON(x86_leaf >= ARRAY_SIZE(reverse_cpuid));
+	BUILD_BUG_ON(reverse_cpuid[x86_leaf].function == 0);
+}
+
+/*
+ * Retrieve the bit mask from an X86_FEATURE_* definition.  Features contain
+ * the hardware defined bit number (stored in bits 4:0) and a software defined
+ * "word" (stored in bits 31:5).  The word is used to index into arrays of
+ * bit masks that hold the per-cpu feature capabilities, e.g. this_cpu_has().
+ */
+static __always_inline u32 bit(int x86_feature)
+{
+	reverse_cpuid_check(x86_feature / 32);
+	return 1 << (x86_feature & 31);
 }
 
 static __always_inline struct cpuid_reg x86_feature_cpuid(unsigned x86_feature)
 {
 	unsigned x86_leaf = x86_feature / 32;
 
-	BUILD_BUG_ON(x86_leaf >= ARRAY_SIZE(reverse_cpuid));
-	BUILD_BUG_ON(reverse_cpuid[x86_leaf].function == 0);
-
+	reverse_cpuid_check(x86_leaf);
 	return reverse_cpuid[x86_leaf];
 }