Re: [PATCH v3 4/5] KVM: SVM: Prevent writes to TSC MSR when Secure TSC is enabled

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

Tom Lendacky <thomas.lendacky@amd.com> writes:

> On 2/17/25 04:22, Nikunj A Dadhania wrote:

>> @@ -3161,6 +3161,20 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>>  
>>  		svm->tsc_aux = data;
>>  		break;
>> +	case MSR_IA32_TSC:
>> +		/*
>> +		 * If Secure TSC is enabled, do not emulate TSC write as TSC calculation
>> +		 * ignores the TSC_OFFSET and TSC_SCALE control fields, record the error
>> +		 * and return a #GP. Allow the TSC to be initialized until the guest state
>> +		 * is protected to prevent unexpected VMM errors.
>> +		 */
>> +		if (vcpu->arch.guest_state_protected && snp_secure_tsc_enabled(vcpu->kvm)) {
>
> I'm not sure if it matters, but do we need to differentiate between
> guest and host write in this situation at all in regards to the message
> or return code?
>

Yes, I think we can have something like the below:

+	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;

>> +			vcpu_unimpl(vcpu, "unimplemented IA32_TSC for secure tsc\n");
>
> s/secure tsc/Secure TSC/ ?
>

Ack,

Thanks
Nikunj