Re: [PATCH v6 03/51] x86/tsc: Ensure that TSC recalibration doesn't run if TSC frequency is known

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Valid virtual machine configurations with legacy CPU models will trigger a spurious WARN_ON_ONCE (and potential kernel panic) due to hypervisor paravirt clocks.
--

--- Patch [3]: [PATCH v6 03/51] x86/tsc: Ensure that TSC recalibration doesn't run if TSC frequency is known ---
commit dd99d8caeb7628be679eca32563aa6618e548028
Author: Sean Christopherson <seanjc@google.com>

x86/tsc: Ensure that TSC recalibration doesn't run if TSC frequency is known

This commit adds a warning and bails from recalibrate_cpu_khz() if
X86_FEATURE_TSC_KNOWN_FREQ is set. This is intended to catch out-of-spec
hardware states on ancient CPUs where recalibration is attempted post-boot.

> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
[ ... ]
> @@ -930,6 +930,9 @@ void recalibrate_cpu_khz(void)
>  	if (!boot_cpu_has(X86_FEATURE_TSC))
>  		return;
>  
> +	if (WARN_ON_ONCE(cpu_feature_enabled(X86_FEATURE_TSC_KNOWN_FREQ)))
> +		return;
> +

[Severity: Medium]
Will this trigger spurious warnings on valid virtual machine configurations?

The commit message mentions that hitting this requires concocting an
impossible CPU model like combining a P4 signature with CPUID 0x15. However,
hypervisor paravirt clocks (like kvmclock, vmware, or mshyperv)
unconditionally call setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ) during
early boot, regardless of the emulated CPU generation.

If a VM is configured with a legacy CPU model (e.g. -cpu athlon) and runs
an SMP=n kernel, the powernow-k7 cpufreq driver can successfully load and
explicitly call recalibrate_cpu_khz(). Since the paravirt clock already
forced the TSC_KNOWN_FREQ feature, cpu_feature_enabled() will evaluate to
true and trigger this WARN_ON_ONCE, which can escalate to a fatal kernel
panic on systems with panic_on_warn enabled.

Does this warning need to account for paravirtualized clocks setting this
feature flag on legacy CPU models?

>  	cpu_khz = x86_platform.calibrate_cpu();
>  	tsc_khz = x86_platform.calibrate_tsc();

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260806233609.212337-1-seanjc@google.com?part=3