RE: [PATCH v5 14/51] x86/tsc: Consolidate forcing of X86_FEATURE_TSC_KNOWN_FREQ for PV code

[Michael Kelley]

From: Sean Christopherson <seanjc@xxxxxxxxxx> Sent: Wednesday, July 1, 2026 12:32 PM
> 
> Now that all paravirt code that explicitly specifies the TSC frequency
> also sets X86_FEATURE_TSC_KNOWN_FREQ, replace all of the one-off code
> and simply set X86_FEATURE_TSC_KNOWN_FREQ if the TSC frequency is known.
> 
> Do NOT force set TSC_KNOWN_FREQ if the "known" TSC frequency was provided
> by the user.  Per commit bd35c77e32e4 ("x86/tsc: Add tsc_early_khz command
> line parameter"), one of the goals of the param is to allow the refined
> calibration work "to do meaningful error checking".
> 
> No functional change intended.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

For the Hyper-V changes,

Reviewed-by: Michael Kelley <mhklinux@xxxxxxxxxxx>

> ---
>  arch/x86/coco/sev/core.c       |  1 -
>  arch/x86/coco/tdx/tdx.c        |  1 -
>  arch/x86/kernel/cpu/acrn.c     |  1 -
>  arch/x86/kernel/cpu/mshyperv.c |  1 -
>  arch/x86/kernel/cpu/vmware.c   |  2 --
>  arch/x86/kernel/jailhouse.c    |  1 -
>  arch/x86/kernel/kvmclock.c     |  1 -
>  arch/x86/kernel/tsc.c          | 13 ++++++++++---
>  arch/x86/xen/time.c            |  1 -
>  9 files changed, 10 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
> index bc5ae9ef74da..72313b36b6f5 100644
> --- a/arch/x86/coco/sev/core.c
> +++ b/arch/x86/coco/sev/core.c
> @@ -2027,7 +2027,6 @@ unsigned int __init snp_secure_tsc_init(void)
> 
>  	secrets = (__force struct snp_secrets_page *)mem;
> 
> -	setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
>  	setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
> 
>  	rdmsrq(MSR_AMD64_GUEST_TSC_FREQ, tsc_freq_mhz);
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index ae2d35f2ef33..94682aca188b 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -1205,7 +1205,6 @@ unsigned int __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);
> 
>  	return info.crystal_khz * info.numerator / info.denominator;
>  }
> diff --git a/arch/x86/kernel/cpu/acrn.c b/arch/x86/kernel/cpu/acrn.c
> index 3818f6ae0629..dc71a6fdd461 100644
> --- a/arch/x86/kernel/cpu/acrn.c
> +++ b/arch/x86/kernel/cpu/acrn.c
> @@ -40,7 +40,6 @@ static void __init acrn_init_platform(void)
>  	if (acrn_tsc_khz_cpuid) {
>  		x86_init.hyper.get_tsc_khz = acrn_get_tsc_khz;
>  		x86_init.hyper.get_cpu_khz = acrn_get_tsc_khz;
> -		setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
>  	}
>  }
> 
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index f9bc1c2d8c93..e03c69a4db33 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -575,7 +575,6 @@ static void __init ms_hyperv_init_platform(void)
>  	    ms_hyperv.misc_features & HV_FEATURE_FREQUENCY_MSRS_AVAILABLE) {
>  		x86_init.hyper.get_tsc_khz = hv_get_tsc_khz;
>  		x86_init.hyper.get_cpu_khz = hv_get_tsc_khz;
> -		setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
>  	}
> 
>  	if (ms_hyperv.priv_high & HV_ISOLATION) {
> diff --git a/arch/x86/kernel/cpu/vmware.c b/arch/x86/kernel/cpu/vmware.c
> index 3cb473cae462..0a3bd90576d4 100644
> --- a/arch/x86/kernel/cpu/vmware.c
> +++ b/arch/x86/kernel/cpu/vmware.c
> @@ -390,8 +390,6 @@ static void __init vmware_set_capabilities(void)
>  {
>  	setup_force_cpu_cap(X86_FEATURE_CONSTANT_TSC);
>  	setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
> -	if (vmware_tsc_khz)
> -		setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
>  	if (vmware_hypercall_mode == CPUID_VMWARE_FEATURES_ECX_VMCALL)
>  		setup_force_cpu_cap(X86_FEATURE_VMCALL);
>  	else if (vmware_hypercall_mode == CPUID_VMWARE_FEATURES_ECX_VMMCALL)
> diff --git a/arch/x86/kernel/jailhouse.c b/arch/x86/kernel/jailhouse.c
> index e24c05ab4fae..ff173052cdce 100644
> --- a/arch/x86/kernel/jailhouse.c
> +++ b/arch/x86/kernel/jailhouse.c
> @@ -255,7 +255,6 @@ static void __init jailhouse_init_platform(void)
>  	pr_debug("Jailhouse: PM-Timer IO Port: %#x\n", pmtmr_ioport);
> 
>  	precalibrated_tsc_khz = setup_data.v1.tsc_khz;
> -	setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
> 
>  	pci_probe = 0;
> 
> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index 4f8299303a19..35a879d33e9e 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -138,7 +138,6 @@ static inline void kvm_sched_clock_init(bool stable)
>   */
>  static unsigned int __init kvm_get_tsc_khz(void)
>  {
> -	setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
>  	return pvclock_tsc_khz(this_cpu_pvti());
>  }
> 
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 1dca9464b41c..676910292af7 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -1541,11 +1541,18 @@ void __init tsc_early_init(void)
>  	if (!known_tsc_khz && x86_init.hyper.get_tsc_khz)
>  		known_tsc_khz = x86_init.hyper.get_tsc_khz();
> 
> +	/*
> +	 * Mark the TSC frequency as known if it was obtained from a hypervisor
> +	 * or trusted firmware.
> +	 */
> +	if (known_tsc_khz)
> +		setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
> +
>  	/*
>  	 * Ignore the user-provided TSC frequency if the exact frequency was
> -	 * obtained from trusted firmware or the hypervisor, as the user-
> -	 * provided frequency is intended as a "starting point", not a known,
> -	 * guaranteed frequency.
> +	 * obtained from trusted firmware or the hypervisor, and don't mark the
> +	 * frequency as known, as the user-provided frequency is intended as a
> +	 * "starting point", not a known, guaranteed frequency
>  	 */
>  	if (!known_tsc_khz)
>  		known_tsc_khz = tsc_early_khz;
> diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
> index 1adb44fdddb2..487ad838c441 100644
> --- a/arch/x86/xen/time.c
> +++ b/arch/x86/xen/time.c
> @@ -43,7 +43,6 @@ static unsigned int __init xen_tsc_khz(void)
>  	struct pvclock_vcpu_time_info *info =
>  		&HYPERVISOR_shared_info->vcpu_info[0].time;
> 
> -	setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
>  	return pvclock_tsc_khz(info);
>  }
> 
> --
> 2.55.0.rc0.799.gd6f94ed593-goog
>