kvm: x86: Report STIBP on GET_SUPPORTED_CPUID

commit d7b09c82 upstream.

Months ago, we have added code to allow direct access to MSR_IA32_SPEC_CTRL
to the guest, which makes STIBP available to guests.  This was implemented
by commits d28b387f ("KVM/VMX: Allow direct access to
MSR_IA32_SPEC_CTRL") and b2ac58f9 ("KVM/SVM: Allow direct access to
MSR_IA32_SPEC_CTRL").

However, we never updated GET_SUPPORTED_CPUID to let userspace know that
STIBP can be enabled in CPUID.  Fix that by updating
kvm_cpuid_8000_0008_ebx_x86_features and kvm_cpuid_7_0_edx_x86_features.
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[bwh: Backported to 4.9: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/kvm/cpuid.c

@@ -356,7 +356,7 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 	/* cpuid 0x80000008.ebx */
 	const u32 kvm_cpuid_8000_0008_ebx_x86_features =
 		F(AMD_IBPB) | F(AMD_IBRS) | F(AMD_SSBD) | F(VIRT_SSBD) |
-		F(AMD_SSB_NO);
+		F(AMD_SSB_NO) | F(AMD_STIBP);
 
 	/* cpuid 0xC0000001.edx */
 	const u32 kvm_cpuid_C000_0001_edx_x86_features =
@@ -381,7 +381,8 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 
 	/* cpuid 7.0.edx*/
 	const u32 kvm_cpuid_7_0_edx_x86_features =
-		F(SPEC_CTRL) | F(SPEC_CTRL_SSBD) | F(ARCH_CAPABILITIES);
+		F(SPEC_CTRL) | F(SPEC_CTRL_SSBD) | F(ARCH_CAPABILITIES) |
+		F(INTEL_STIBP);
 
 	/* all calls to cpuid_count() should be made on the same cpu */
 	get_cpu();