[PATCH 1/2] KVM: x86: Add build-time assertion on usage of bit()

[Sean Christopherson]

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().

Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---

Note, the premature newline in the first line of the second comment is
intentional to reduce churn in the next patch.

 arch/x86/kvm/x86.h | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index cab5e71f0f0f..4ee4175c66a7 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -144,9 +144,28 @@ static inline bool is_pae_paging(struct kvm_vcpu *vcpu)
 	return !is_long_mode(vcpu) && is_pae(vcpu) && is_paging(vcpu);
 }
 
-static inline u32 bit(int bitno)
+/*
+ * 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 feature)
 {
-	return 1 << (bitno & 31);
+	/*
+	 * bit() is intended to be used only for hardware-defined
+	 * words, i.e. words whose bits directly correspond to a CPUID leaf.
+	 * Retrieving the bit mask 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.
+	 */
+	BUILD_BUG_ON((feature >> 5) == CPUID_LNX_1);
+	BUILD_BUG_ON((feature >> 5) == CPUID_LNX_2);
+	BUILD_BUG_ON((feature >> 5) == CPUID_LNX_3);
+	BUILD_BUG_ON((feature >> 5) == CPUID_LNX_4);
+	BUILD_BUG_ON((feature >> 5) > CPUID_7_EDX);
+
+	return 1 << (feature & 31);
 }
 
 static inline u8 vcpu_virt_addr_bits(struct kvm_vcpu *vcpu)
-- 
2.24.0