KVM: x86: Add speculative control CPUID support for guests

CVE-2017-5715 (Spectre v2 Intel)

Provide the guest with the speculative control CPUID related values.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
(backported from commit cbfe052b7e811a2854162b210f242d3e815cbc17)
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

arch/x86/kvm/cpuid.c

@@ -65,6 +65,10 @@ u64 kvm_supported_xcr0(void)
 
 #define F(x) bit(X86_FEATURE_##x)
 
+/* These are scattered features in cpufeatures.h. */
+#define KVM_CPUID_BIT_SPEC_CTRL		26
+#define KF(x) bit(KVM_CPUID_BIT_##x)
+
 int kvm_update_cpuid(struct kvm_vcpu *vcpu)
 {
 	struct kvm_cpuid_entry2 *best;
@@ -361,6 +365,10 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 	const u32 kvm_supported_word10_x86_features =
 		F(XSAVEOPT) | F(XSAVEC) | F(XGETBV1) | f_xsaves;
 
+	/* cpuid 0x80000008.0.ebx */
+	const u32 kvm_cpuid_80000008_0_ebx_x86_features =
+		F(IBPB);
+
 	/* all calls to cpuid_count() should be made on the same cpu */
 	get_cpu();
 
@@ -586,7 +594,9 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 		if (!g_phys_as)
 			g_phys_as = phys_as;
 		entry->eax = g_phys_as | (virt_as << 8);
-		entry->ebx = entry->edx = 0;
+		entry->ebx &= kvm_cpuid_80000008_0_ebx_x86_features;
+		cpuid_mask(&entry->ebx, 13 /* CPUID_8000_0008_EBX */);
+		entry->edx = 0;
 		break;
 	}
 	case 0x80000019: