Re: [PATCH 06/16] x86/tdx: Override PV calibration routines with CPUID-based calibration

[Nikunj A Dadhania <nikunj@amd.com>]

Sean Christopherson <seanjc@google.com> writes:

> When running as a TDX guest, explicitly override the TSC frequency
> calibration routine with CPUID-based calibration instead of potentially
> relying on a hypervisor-controlled PV routine.  For TDX guests, CPUID.0x15
> is always emulated by the TDX-Module, i.e. the information from CPUID is
> more trustworthy than the information provided by the hypervisor.
>
> To maintain backwards compatibility with TDX guest kernels that use native
> calibration, and because it's the least awful option, retain
> native_calibrate_tsc()'s stuffing of the local APIC bus period using the
> core crystal frequency.  While it's entirely possible for the hypervisor
> to emulate the APIC timer at a different frequency than the core crystal
> frequency, the commonly accepted interpretation of Intel's SDM is that APIC
> timer runs at the core crystal frequency when that latter is enumerated via
> CPUID:
>
>   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).
>
> If the hypervisor is malicious and deliberately runs the APIC timer at the
> wrong frequency, nothing would stop the hypervisor from modifying the
> frequency at any time, i.e. attempting to manually calibrate the frequency
> out of paranoia would be futile.
>
> Deliberately leave the CPU frequency calibration routine as is, since the
> TDX-Module doesn't provide any guarantees with respect to CPUID.0x16.

Does TDX use kvmclock? If yes, kvmclock would have registered the CPU
frequency calibration routine:

	tsc_register_calibration_routines(kvm_get_tsc_khz, kvm_get_cpu_khz,
 					  tsc_properties);

so TDX will use kvm_get_cpu_khz(), which will either use CPUID.0x16 or
PV clock, is this on the expected line ?

Regards
Nikunj

> +
> +void __init tdx_tsc_init(void)
> +{
> +	/* TSC is the only reliable clock in TDX guest */
> +	setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
> +	setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
> +
> +	/*
> +	 * Override the PV calibration routines (if set) with more trustworthy
> +	 * CPUID-based calibration.  The TDX module emulates CPUID, whereas any
> +	 * PV information is provided by the hypervisor.
> +	 */
> +	tsc_register_calibration_routines(tdx_get_tsc_khz, NULL);
> +}