KVM: x86: Fix BUILD_BUG() in __cpuid_entry_get_reg() w/ CONFIG_UBSAN=y

Take the target reg in __cpuid_entry_get_reg() instead of a pointer to a
struct cpuid_reg.  When building with -fsanitize=alignment (enabled by
CONFIG_UBSAN=y), some versions of gcc get tripped up on the pointer and
trigger the BUILD_BUG().
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Fixes: d8577a4c ("KVM: x86: Do host CPUID at load time to mask KVM cpu caps")
Fixes: 4c61534a ("KVM: x86: Introduce cpuid_entry_{get,has}() accessors")
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200325191259.23559-1-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/cpuid.c

@@ -269,7 +269,7 @@ static __always_inline void kvm_cpu_cap_mask(enum cpuid_leafs leaf, u32 mask)
 	cpuid_count(cpuid.function, cpuid.index,
 		    &entry.eax, &entry.ebx, &entry.ecx, &entry.edx);
 
-	kvm_cpu_caps[leaf] &= *__cpuid_entry_get_reg(&entry, &cpuid);
+	kvm_cpu_caps[leaf] &= *__cpuid_entry_get_reg(&entry, cpuid.reg);
 }
 
 void kvm_set_cpu_caps(void)

arch/x86/kvm/cpuid.h

@@ -99,9 +99,9 @@ static __always_inline struct cpuid_reg x86_feature_cpuid(unsigned int x86_featu
 }
 
 static __always_inline u32 *__cpuid_entry_get_reg(struct kvm_cpuid_entry2 *entry,
-						  const struct cpuid_reg *cpuid)
+						  u32 reg)
 {
-	switch (cpuid->reg) {
+	switch (reg) {
 	case CPUID_EAX:
 		return &entry->eax;
 	case CPUID_EBX:
@@ -121,7 +121,7 @@ static __always_inline u32 *cpuid_entry_get_reg(struct kvm_cpuid_entry2 *entry,
 {
 	const struct cpuid_reg cpuid = x86_feature_cpuid(x86_feature);
 
-	return __cpuid_entry_get_reg(entry, &cpuid);
+	return __cpuid_entry_get_reg(entry, cpuid.reg);
 }
 
 static __always_inline u32 cpuid_entry_get(struct kvm_cpuid_entry2 *entry,
@@ -189,7 +189,7 @@ static __always_inline u32 *guest_cpuid_get_register(struct kvm_vcpu *vcpu,
 	if (!entry)
 		return NULL;
 
-	return __cpuid_entry_get_reg(entry, &cpuid);
+	return __cpuid_entry_get_reg(entry, cpuid.reg);
 }
 
 static __always_inline bool guest_cpuid_has(struct kvm_vcpu *vcpu,