Messages in this thread | | | From | Vishal Annapurve <> | Date | Sat, 21 Oct 2023 07:24:55 +0530 | Subject | Re: [PATCH] x86/tdx: Override the tsc calibration for TDX VMs |
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On Sat, Oct 14, 2023 at 4:32 AM Sean Christopherson <seanjc@google.com> wrote: > > On Fri, Oct 13, 2023, Sean Christopherson wrote: > > On Fri, Oct 06, 2023, Vishal Annapurve wrote: > > > TSC calibration for native execution gets the TSC frequency from CPUID, > > > but also ends up setting lapic_timer_period. When using oneshot mode > > > with lapic timer, predefined value of lapic_timer_period causes lapic > > > timer calibration to be skipped with wrong multipliers set for lapic > > > timer. > > > > > > To avoid this issue, override the TSC calibration step for TDX VMs to > > > just calculate the TSC frequency using cpuid values. > > > > This is a hack to workaround a KVM TDX bug. Per Intel's SDM: > > > > The APIC timer frequency will be the processor’s bus clock or core crystal > > clock frequency (when TSC/core crystal clock ratio is enumerated in CPUID > > leaf 0x15) divided by the value specified in the divide configuration register. > > > > TDX hardcodes the core crystal frequency to 25Mhz, whereas KVM hardcodes the APIC > > bus frequency to 1Ghz. Upstream KVM's *current* behavior is fine, because KVM > > doesn't advertise support for CPUID 0x15, i.e. doesn't announce to host userspace > > that it's safe to expose CPUID 0x15 to the guest. Because TDX makes exposing > > CPUID 0x15 mandatory, KVM needs to be taught to correctly emulate the guest's > > APIC bus frequency, a.k.a. the TDX guest core crystal frequency of 25Mhz. > >
Ack, makes sense to pursue this fix from the KVM TDX side instead of guest changes as per your suggestion.
> > > > I.e. tmict_to_ns() needs to replace APIC_BUS_CYCLE_NS with some math that makes > > the guest's APIC timer actually run at 25Mhz given whatever the host APIC bus > > runs at. > > > > static inline u64 tmict_to_ns(struct kvm_lapic *apic, u32 tmict) > > { > > return (u64)tmict * APIC_BUS_CYCLE_NS * (u64)apic->divide_count; > > } >
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