Hi Sean,
On 10/2/23 18:49, Sean Christopherson wrote:
> On Mon, Oct 02, 2023, Dongli Zhang wrote:
>>> @@ -12185,6 +12203,10 @@ int kvm_arch_hardware_enable(void)
>>> if (ret != 0)
>>> return ret;
>>>
>>> + if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
>>> + kvm_get_time_scale(NSEC_PER_SEC, tsc_khz * 1000LL,
>>> + &host_tsc_shift, &host_tsc_to_system_mul);
>>
>> I agree that to use the kvmclock to calculate the ns elapsed when updating the
>> master clock.
>>
>> Would you take the tsc scaling into consideration?
>>
>> While the host_tsc_shift and host_tsc_to_system_mul are pre-computed, how about
>> the VM using different TSC frequency?
>
> Heh, I'm pretty sure that's completely broken today. I don't see anything in KVM
> that takes hardware TSC scaling into account.
>
> This code:
>
> if (unlikely(vcpu->hw_tsc_khz != tgt_tsc_khz)) {
> kvm_get_time_scale(NSEC_PER_SEC, tgt_tsc_khz * 1000LL,
> &vcpu->hv_clock.tsc_shift,
> &vcpu->hv_clock.tsc_to_system_mul);
> vcpu->hw_tsc_khz = tgt_tsc_khz;
> kvm_xen_update_tsc_info(v);
> }
>
> is recomputing the multipler+shift for the current *physical* CPU, it's not
> related to the guest's TSC in any way.
The below is the code.
line 3175: query freq for current *physical* CPU.
line 3211: scale the freq if scaling is involved.
line 3215: compute the view for guest based on new 'tgt_tsc_khz' after scaling.
3146 static int kvm_guest_time_update(struct kvm_vcpu *v)
3147 {
3148 unsigned long flags, tgt_tsc_khz;
3149 unsigned seq;
... ...
3173 /* Keep irq disabled to prevent changes to the clock */
3174 local_irq_save(flags);
3175 tgt_tsc_khz = get_cpu_tsc_khz();
... ...
3210 if (kvm_caps.has_tsc_control)
3211 tgt_tsc_khz = kvm_scale_tsc(tgt_tsc_khz,
3212 v->arch.l1_tsc_scaling_ratio);
3213
3214 if (unlikely(vcpu->hw_tsc_khz != tgt_tsc_khz)) {
3215 kvm_get_time_scale(NSEC_PER_SEC, tgt_tsc_khz * 1000LL,
3216 &vcpu->hv_clock.tsc_shift,
3217 &vcpu->hv_clock.tsc_to_system_mul);
3218 vcpu->hw_tsc_khz = tgt_tsc_khz;
3219 kvm_xen_update_tsc_info(v);
3220 }
Would you please let me know if the above understanding is incorrect?
Thank you very much!
Dongli Zhang
>
> __get_kvmclock() again shows that quite clearly, there's no scaling for the guest
> TSC anywhere in there.
