KVM: x86: Add support for reverse CPUID lookup of scattered features

Introduce a scheme that allows KVM's CPUID magic to support features
that are scattered in the kernel's feature words.  To advertise and/or
query guest support for CPUID-based features, KVM requires the bit
number of an X86_FEATURE_* to match the bit number in its associated
CPUID entry.  For scattered features, this does not hold true.

Add a framework to allow defining KVM-only words, stored in
kvm_cpu_caps after the shared kernel caps, that can be used to gather
the scattered feature bits by translating X86_FEATURE_* flags into their
KVM-defined feature.

Note, because reverse_cpuid_check() effectively forces kvm_cpu_caps
lookups to be resolved at compile time, there is no runtime cost for
translating from kernel-defined to kvm-defined features.

More details here:  https://lkml.kernel.org/r/X/jxCOLG+HUO4QlZ@google.com

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Kai Huang <kai.huang@intel.com>
Message-Id: <16cad8d00475f67867fb36701fc7fb7c1ec86ce1.1618196135.git.kai.huang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 34 insertions, 5 deletions

diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
index ded84d244f19..315fa45eb7c8 100644
--- a/arch/x86/kvm/cpuid.h
+++ b/arch/x86/kvm/cpuid.h
@@ -7,7 +7,20 @@
 #include <asm/processor.h>
 #include <uapi/asm/kvm_para.h>
 
-extern u32 kvm_cpu_caps[NCAPINTS] __read_mostly;
+/*
+ * 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.
+ */
+enum kvm_only_cpuid_leafs {
+	NR_KVM_CPU_CAPS = NCAPINTS,
+
+	NKVMCAPINTS = NR_KVM_CPU_CAPS - NCAPINTS,
+};
+
+#define X86_KVM_FEATURE(w, f)		((w)*32 + (f))
+
+extern u32 kvm_cpu_caps[NR_KVM_CPU_CAPS] __read_mostly;
 void kvm_set_cpu_caps(void);
 
 void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu);
@@ -101,6 +114,20 @@ static __always_inline void reverse_cpuid_check(unsigned int x86_leaf)
 }
 
 /*
+ * Translate feature bits that are scattered in the kernel's cpufeatures word
+ * into KVM feature words that align with hardware's definitions.
+ */
+static __always_inline u32 __feature_translate(int x86_feature)
+{
+	return x86_feature;
+}
+
+static __always_inline u32 __feature_leaf(int x86_feature)
+{
+	return __feature_translate(x86_feature) / 32;
+}
+
+/*
  * 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
@@ -108,6 +135,8 @@ static __always_inline void reverse_cpuid_check(unsigned int x86_leaf)
  */
 static __always_inline u32 __feature_bit(int x86_feature)
 {
+	x86_feature = __feature_translate(x86_feature);
+
 	reverse_cpuid_check(x86_feature / 32);
 	return 1 << (x86_feature & 31);
 }
@@ -116,7 +145,7 @@ static __always_inline u32 __feature_bit(int x86_feature)
 
 static __always_inline struct cpuid_reg x86_feature_cpuid(unsigned int x86_feature)
 {
-	unsigned int x86_leaf = x86_feature / 32;
+	unsigned int x86_leaf = __feature_leaf(x86_feature);
 
 	reverse_cpuid_check(x86_leaf);
 	return reverse_cpuid[x86_leaf];
@@ -316,7 +345,7 @@ static inline bool cpuid_fault_enabled(struct kvm_vcpu *vcpu)
 
 static __always_inline void kvm_cpu_cap_clear(unsigned int x86_feature)
 {
-	unsigned int x86_leaf = x86_feature / 32;
+	unsigned int x86_leaf = __feature_leaf(x86_feature);
 
 	reverse_cpuid_check(x86_leaf);
 	kvm_cpu_caps[x86_leaf] &= ~__feature_bit(x86_feature);
@@ -324,7 +353,7 @@ static __always_inline void kvm_cpu_cap_clear(unsigned int x86_feature)
 
 static __always_inline void kvm_cpu_cap_set(unsigned int x86_feature)
 {
-	unsigned int x86_leaf = x86_feature / 32;
+	unsigned int x86_leaf = __feature_leaf(x86_feature);
 
 	reverse_cpuid_check(x86_leaf);
 	kvm_cpu_caps[x86_leaf] |= __feature_bit(x86_feature);
@@ -332,7 +361,7 @@ static __always_inline void kvm_cpu_cap_set(unsigned int x86_feature)
 
 static __always_inline u32 kvm_cpu_cap_get(unsigned int x86_feature)
 {
-	unsigned int x86_leaf = x86_feature / 32;
+	unsigned int x86_leaf = __feature_leaf(x86_feature);
 
 	reverse_cpuid_check(x86_leaf);
 	return kvm_cpu_caps[x86_leaf] & __feature_bit(x86_feature);