[PATCH v4 4/5] KVM: SVM: Prevent writes to TSC MSR when Secure TSC is enabled

[Nikunj A Dadhania <nikunj@amd.com>]

Disallow writes to MSR_IA32_TSC for Secure TSC enabled SNP guests. Even if
KVM attempts to emulate such writes, TSC calculation will ignore the
TSC_SCALE and TSC_OFFSET present in the VMCB. Instead, it will use
GUEST_TSC_SCALE and GUEST_TSC_OFFSET stored in the VMSA.

Additionally, incorporate a check for protected guest state to allow the
VMM to initialize the TSC MSR.

Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
---
 arch/x86/kvm/svm/svm.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index e65721db1f81..1652848b0240 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3161,6 +3161,25 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 
 		svm->tsc_aux = data;
 		break;
+	case MSR_IA32_TSC:
+		/*
+		 * For Secure TSC enabled VM, do not emulate TSC write as the
+		 * TSC calculation ignores the TSC_OFFSET and TSC_SCALE control
+		 * fields.
+		 *
+		 * Guest writes: Record the error and return a #GP.
+		 * Host writes are ignored.
+		 */
+		if (snp_secure_tsc_enabled(vcpu->kvm)) {
+			if (!msr->host_initiated) {
+				vcpu_unimpl(vcpu, "unimplemented IA32_TSC for Secure TSC\n");
+				return 1;
+			} else
+				return 0;
+		}
+
+		ret = kvm_set_msr_common(vcpu, msr);
+		break;
 	case MSR_IA32_DEBUGCTLMSR:
 		if (!lbrv) {
 			kvm_pr_unimpl_wrmsr(vcpu, ecx, data);
-- 
2.43.0