On 10/24/25 12:16, Naveen N Rao wrote:
> On Fri, Oct 24, 2025 at 10:00:08AM -0500, Tom Lendacky wrote:
>> On 10/8/25 04:52, Naveen N Rao wrote:
>>> On Tue, Oct 07, 2025 at 08:31:47AM -0500, Tom Lendacky wrote:
>>>> On 9/25/25 05:17, Naveen N Rao (AMD) wrote:
>>>
>>> ...
>>>
>>>>> +
>>>>> +static void
>>>>> +sev_snp_guest_set_tsc_frequency(Object *obj, Visitor *v, const char
>>>>> *name,
>>>>> + void *opaque, Error **errp)
>>>>> +{
>>>>> + uint32_t value;
>>>>> +
>>>>> + if (!visit_type_uint32(v, name, &value, errp)) {
>>>>> + return;
>>>>> + }
>>>>> +
>>>>> + SEV_SNP_GUEST(obj)->tsc_khz = value / 1000;
>>>>
>>>> This will cause a value that isn't evenly divisible by 1000 to be
>>>> rounded down, e.g.: tsc-frequency=2500000999. Should this name instead
>>>> just be tsc-khz or secure-tsc-khz (to show it is truly associated with
>>>> Secure TSC)?
>>>
>>> I modeled this after the existing tsc-frequency parameter on the cpu
>>> object to keep it simple (parameter is the same, just where it is
>>> specified differs). This also aligns with TDX which re-uses the
>>> tsc-frequency parameter on the cpu object.
>>
>> So why aren't we using the one on the cpu object instead of creating a
>> duplicate parameter? There should be some way to get that value, no?
>
> I had spent some time on this, but I couldn't figure out a simple way to
> make that work.
>
> TDX uses a vcpu pre-create hook (similar to KVM) to get access to and
> set the TSC value from the cpu object. For SEV-SNP, we need the TSC
> frequency during SNP_LAUNCH_START which is quite early and we don't have
> access to the cpu object there.
>
> Admittedly, my qemu understanding is limited. If there is a way to
> re-use the cpu tsc-frequency field, then that would be ideal.
>
> Any ideas/suggestions?
Any Qemu experts know how the SEV support would be able to access the
TSC value from the -cpu command line option at LAUNCH time?
Thanks,
Tom
>
>
> Thanks,
> Naveen
>
