On Mon, Jun 08 2026 at 15:38, Sean Christopherson wrote:
> On Sat, Jun 06, 2026, David Woodhouse wrote:
>> > Along with:
>> >
>> > if (!hypervisor_is_type(X86_HYPER_NATIVE)) {
>> > if (tsc_khz_early)
>> > pr_warn("Ignoring non-sensical tsc_early_khz command line
>> > argument\n");
>> >
>> > or something daft like that.
>
> Ya, I ended up in the same place once Sashiko pointed out that skipping the
> SNP/TDX
> setup was hazardous[*], and also once I realized that tsc_khz_early
> *complemented*
> the refinement instead of replacing it.
>
> This is what I have locally:
>
> if (cc_platform_has(CC_ATTR_GUEST_SNP_SECURE_TSC))
> known_tsc_khz = snp_secure_tsc_init();
> else if (boot_cpu_has(X86_FEATURE_TDX_GUEST))
> known_tsc_khz = tdx_tsc_init();
>
> /*
> * If the TSC frequency wasn't provided by trusted firmware, try to
> get
> * it from the hypervisor (which is untrusted when running as a CoCo
> guest).
> */
> 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. Don't mark the frequency as known if the user
> * specified the frequency, as the user-provided frequency is intended
> * as a "starting point", not a known, guaranteed frequency.
> */
> if (known_tsc_khz && !tsc_early_khz)
> setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
If the frequenct is known via the above then you want to set the
KNOWN_FREQ feature bit unconditionally. SNP/TDX/hypervisor override the
command line argument as you print below.
> /*
> * 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.
> */
> if (!known_tsc_khz)
> known_tsc_khz = tsc_early_khz;
> else if (tsc_early_khz)
> pr_err("Ignoring 'tsc_early_khz' in favor of
> firmware/hypervisor.\n");
>> All the nonsense about updating it every time we enter a CPU could just
>> go away completely.
>
> But to Thomas' point, why bother? For actual old hardware, kvmclock is what
> it
> is. For modern hardware, it's completely antiquated.
I agree, but we are not forced to make it a first class citizen to the
detriment of sane systems.
Thanks,
tglx
