Re: [PATCH v4 5/5] KVM: SVM: Enable Secure TSC for SNP guests

Tom Lendacky
2025-03-11T14:33:24Z

On 3/11/25 06:05, Nikunj A. Dadhania wrote:
> On 3/11/2025 2:41 PM, Nikunj A. Dadhania wrote:
>> On 3/10/2025 9:09 PM, Tom Lendacky wrote:
>>> On 3/10/25 01:45, Nikunj A Dadhania wrote:
>>
>>>> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
>>>> index 50263b473f95..b61d6bd75b37 100644
>>>> --- a/arch/x86/kvm/svm/sev.c
>>>> +++ b/arch/x86/kvm/svm/sev.c
>>>> @@ -2205,6 +2205,20 @@ static int snp_launch_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
>>>>
>>>> start.gctx_paddr = __psp_pa(sev->snp_context);
>>>> start.policy = params.policy;
>>>> +
>>>> + if (snp_secure_tsc_enabled(kvm)) {
>>>> + u32 user_tsc_khz = params.desired_tsc_khz;
>>>> +
>>>> + /* Use tsc_khz if the VMM has not provided the TSC frequency */
>>>> + if (!user_tsc_khz)
>>>> + user_tsc_khz = tsc_khz;
>>>> +
>>>> + start.desired_tsc_khz = user_tsc_khz;
>>>
>>> Do we need to perform any sanity checking against this value?
>>
>> On the higher side, sev-snp-guest.stsc-freq is u32, a Secure TSC guest boots fine with
>> TSC frequency set to the highest value (stsc-freq=0xFFFFFFFF).
>>
>> On the lower side as MSR_AMD64_GUEST_TSC_FREQ is in MHz, TSC clock should at least be 1Mhz.
>
> Something like this ?
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index b61d6bd75b37..c46b6afa969d 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2213,6 +2213,14 @@ static int snp_launch_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
> if (!user_tsc_khz)
> user_tsc_khz = tsc_khz;
>
> + /*
> + * The minimum granularity for reporting Secure TSC frequency is
> + * 1MHz. Return an error if the user specifies a TSC frequency
> + * less than 1MHz.
> + */
> + if (user_tsc_khz < 1000)
> + return -EINVAL;

Seems reasonable to me. I'll let Sean or Paolo weigh in on it. I don't
think we need a message, there should be a check in the VMM, too, which
would be able to provide information to the end user?

Thanks,
Tom

> +
> start.desired_tsc_khz = user_tsc_khz;
>
> /* Set the arch default TSC for the VM*/
>
> Regards
> Nikunj