Re: [PATCH 8/8] target/i386: SEV: Add support for setting TSC frequency for Secure TSC

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

On 9/20/2025 3:36 AM, Tom Lendacky wrote:
> On 9/18/25 05:27, Naveen N Rao (AMD) wrote:

>> @@ -1085,6 +1093,18 @@ sev_snp_launch_start(SevCommonState *sev_common)
>>              return 1;
>>      }
>>  
>> +    if (is_sev_feature_set(sev_common, SVM_SEV_FEAT_SECURE_TSC)) {
>> +        rc = -EINVAL;
>> +        if (kvm_check_extension(kvm_state, KVM_CAP_VM_TSC_CONTROL)) {
>> +            rc = kvm_vm_ioctl(kvm_state, KVM_SET_TSC_KHZ, sev_snp_guest->tsc_khz);
>> +        }
>> +        if (rc < 0) {
>> +            error_report("%s: Unable to set Secure TSC frequency to %u kHz ret=%d",
>> +                         __func__, sev_snp_guest->tsc_khz, rc);
>> +            return 1;
>> +        }
> 
> It looks like KVM_CAP_VM_TSC_CONTROL is required for Secure TSC. Should
> this cap check be part of check_sev_features() then, rather than waiting
> until launch start?

If the user has not provided tsc-frequency, KVM_CAP_VM_TSC_CONTROL is not required.

> 
> And does KVM_SET_TSC_KHZ have to be called if "tsc-frequency" wasn't set?
No, this is not required. This patch has changed a bit from my original version, we should have something like below: 

if (is_sev_feature_set(sev_common, SVM_SEV_FEAT_SECURE_TSC) && sev_snp_guest->stsc_khz) {
...
}

Regards
Nikunj