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

Nikunj A. Dadhania
2025-03-11T15:34:09Z

On 3/11/2025 8:22 PM, Sean Christopherson wrote:
> On Tue, Mar 11, 2025, Tom Lendacky wrote:
>> 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;
>
> The code just below this clobbers kvm->arch.default_tsc_khz, which could already
> have been set by userspace. Why? Either require params.desired_tsc_khz to match
> kvm->arch.default_tsc_khz, or have KVM's ABI be that KVM stuffs desired_tsc_khz
> based on kvm->arch.default_tsc_khz. I don't see any reason to add yet another
> way to control TSC.

Setting of the desired TSC frequency needs to be done during SNP_LAUNCH_START,
while parsing of the tsc-frequency happens as part of the cpu common class,
and kvm->arch.default_tsc_khz is set pretty late.

Intially, I had planned using "-cpu model,tsc-frequency=<>", but couldn't
find a way to get this parsed early during SNP_LAUNCH_START.

Regards,
Nikunj