KVM: VMX: Fix conditions for guest IA32_XSS support

Volume 4 of the SDM says that IA32_XSS is supported
if CPUID(EAX=0DH,ECX=1):EAX.XSS[bit 3] is set, so only the
X86_FEATURE_XSAVES check is necessary (X86_FEATURE_XSAVES is the Linux
name for CPUID(EAX=0DH,ECX=1):EAX.XSS[bit 3]).

Fixes: 4d763b16 ("KVM: VMX: check CPUID before allowing read/write of IA32_XSS")
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Change-Id: I9059b9f2e3595e4b09a4cdcf14b933b22ebad419
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -1830,10 +1830,8 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		return vmx_get_vmx_msr(&vmx->nested.msrs, msr_info->index,
 				       &msr_info->data);
 	case MSR_IA32_XSS:
-		if (!vmx_xsaves_supported() ||
-		    (!msr_info->host_initiated &&
-		     !(guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) &&
-		       guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))))
+		if (!msr_info->host_initiated &&
+		    !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
 			return 1;
 		msr_info->data = vcpu->arch.ia32_xss;
 		break;
@@ -2073,10 +2071,8 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 			return 1;
 		return vmx_set_vmx_msr(vcpu, msr_index, data);
 	case MSR_IA32_XSS:
-		if (!vmx_xsaves_supported() ||
-		    (!msr_info->host_initiated &&
-		     !(guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) &&
-		       guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))))
+		if (!msr_info->host_initiated &&
+		    !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
 			return 1;
 		/*
 		 * The only supported bit as of Skylake is bit 8, but
@@ -2085,11 +2081,13 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		if (data != 0)
 			return 1;
 		vcpu->arch.ia32_xss = data;
+		if (vcpu->arch.xsaves_enabled) {
 			if (vcpu->arch.ia32_xss != host_xss)
 				add_atomic_switch_msr(vmx, MSR_IA32_XSS,
 					vcpu->arch.ia32_xss, host_xss, false);
 			else
 				clear_atomic_switch_msr(vmx, MSR_IA32_XSS);
+		}
 		break;
 	case MSR_IA32_RTIT_CTL:
 		if ((pt_mode != PT_MODE_HOST_GUEST) ||