Re: [PATCH v6 08/51] x86/sev: Shove SNP's secure/trusted TSC frequency directly into "calibration"

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

On Thu, 2026-08-06 at 16:35 -0700, Sean Christopherson wrote:
> As a first step towards dropping .calibrate_{cpu,tsc}() and explicitly
> defining precedence/priority for "calibration" routines, pass the secure
> TSC frequency obtained from SNP firmware directly to
> determine_cpu_tsc_frequencies() instead of overriding the .calibrate_tsc()
> hook.
>
> Unlike the native calibration routines, all of the paravirtual overrides,
> including SNP and TDX, are constant in the sense that the frequency
> provided by the hypervisor or trusted firmware is fixed, known, and always
> available during early boot.  More importantly, for CoCo (SNP and TDX) VMs,
> it's imperative that the kernel uses the frequency provided by the trusted
> firmware, not by the untrusted hypervisor.  Enforcing the priority between
> sources by carefully ordering seemingly unrelated init calls, so that the
> trusted override "wins", is brittle and all but impossible to follow.
>
> Explicitly ignore tsc_early_khz if the exact TSC frequency was obtained
> from trusted firmware, as per commit bd35c77e32e4 ("x86/tsc: Add
> tsc_early_khz command line parameter"), the goal of the param is to play
> nice with setups that provide partial frequency information in CPUID, i.e.
> is NOT intended to be a hard override.  Neither SNP's secure TSC nor TDX
> was supported when commit bd35c77e32e4 landed back in 2020, i.e. lack of
> consideration for the interaction was purely due to oversight when SNP and
> TDX support came along.
>
> Reviewed-by: Nikunj A Dadhania <nikunj@xxxxxxx>
> Tested-by: Nikunj A Dadhania <nikunj@xxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: David Woodhouse <dwmw@xxxxxxxxxxxx>

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