KVM: x86: Update KVM-only leaf handling to allow for 100% KVM-only leafs

Rename kvm_cpu_cap_init_scattered() to kvm_cpu_cap_init_kvm_defined() in
anticipation of adding KVM-only CPUID leafs that aren't recognized by the
kernel and thus not scattered, i.e. for leafs that are 100% KVM-defined.

Adjust/add comments to kvm_only_cpuid_leafs and KVM_X86_FEATURE to
document how to create new kvm_only_cpuid_leafs entries for scattered
features as well as features that are entirely unknown to the kernel.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221125125845.1182922-3-jiaxi.chen@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/cpuid.c

@@ -549,9 +549,9 @@ static __always_inline void __kvm_cpu_cap_mask(unsigned int leaf)
 }
 
 static __always_inline
-void kvm_cpu_cap_init_scattered(enum kvm_only_cpuid_leafs leaf, u32 mask)
+void kvm_cpu_cap_init_kvm_defined(enum kvm_only_cpuid_leafs leaf, u32 mask)
 {
-	/* Use kvm_cpu_cap_mask for non-scattered leafs. */
+	/* Use kvm_cpu_cap_mask for leafs that aren't KVM-only. */
 	BUILD_BUG_ON(leaf < NCAPINTS);
 
 	kvm_cpu_caps[leaf] = mask;
@@ -561,7 +561,7 @@ void kvm_cpu_cap_init_scattered(enum kvm_only_cpuid_leafs leaf, u32 mask)
 
 static __always_inline void kvm_cpu_cap_mask(enum cpuid_leafs leaf, u32 mask)
 {
-	/* Use kvm_cpu_cap_init_scattered for scattered leafs. */
+	/* Use kvm_cpu_cap_init_kvm_defined for KVM-only leafs. */
 	BUILD_BUG_ON(leaf >= NCAPINTS);
 
 	kvm_cpu_caps[leaf] &= mask;
@@ -670,7 +670,7 @@ void kvm_set_cpu_caps(void)
 		F(XSAVEOPT) | F(XSAVEC) | F(XGETBV1) | F(XSAVES) | f_xfd
 	);
 
-	kvm_cpu_cap_init_scattered(CPUID_12_EAX,
+	kvm_cpu_cap_init_kvm_defined(CPUID_12_EAX,
 		SF(SGX1) | SF(SGX2)
 	);
 

arch/x86/kvm/reverse_cpuid.h

@@ -7,9 +7,9 @@
 #include <asm/cpufeatures.h>
 
 /*
- * Hardware-defined CPUID leafs that are scattered in the kernel, but need to
- * be directly used by KVM.  Note, these word values conflict with the kernel's
- * "bug" caps, but KVM doesn't use those.
+ * Hardware-defined CPUID leafs that are either scattered by the kernel or are
+ * unknown to the kernel, but need to be directly used by KVM.  Note, these
+ * word values conflict with the kernel's "bug" caps, but KVM doesn't use those.
  */
 enum kvm_only_cpuid_leafs {
 	CPUID_12_EAX	 = NCAPINTS,
@@ -18,6 +18,18 @@ enum kvm_only_cpuid_leafs {
 	NKVMCAPINTS = NR_KVM_CPU_CAPS - NCAPINTS,
 };
 
+/*
+ * Define a KVM-only feature flag.
+ *
+ * For features that are scattered by cpufeatures.h, __feature_translate() also
+ * needs to be updated to translate the kernel-defined feature into the
+ * KVM-defined feature.
+ *
+ * For features that are 100% KVM-only, i.e. not defined by cpufeatures.h,
+ * forego the intermediate KVM_X86_FEATURE and directly define X86_FEATURE_* so
+ * that X86_FEATURE_* can be used in KVM.  No __feature_translate() handling is
+ * needed in this case.
+ */
 #define KVM_X86_FEATURE(w, f)		((w)*32 + (f))
 
 /* Intel-defined SGX sub-features, CPUID level 0x12 (EAX). */