On Fri, Jun 05, 2026, Thomas Gleixner wrote:
> On Fri, Jun 05 2026 at 11:04, Sean Christopherson wrote:
> But we also should have a check in the TSC init code somewhere which
> validates that X86_FEATURE_CONSTANT_TSC is set when
> X86_FEATURE_TSC_KNOWN_FREQ is set. X86_FEATURE_TSC_KNOWN_FREQ is useless
> w/o X86_FEATURE_CONSTANT_TSC.
Ugh, any objection to punting on this for now? KVM and Xen guests will trigger
TSC_KNOWN_FREQ without CONSTANT_TSC, thanks to commits:
e10f78050323 ("kvmclock: fix TSC calibration for nested guests")
898ec52d2ba0 ("x86/xen/time: Set the X86_FEATURE_TSC_KNOWN_FREQ flag in
xen_tsc_khz()")
Hyper-V guests might as well? Hyper-V's handling of TSC is weird, even for a
hypervisor.
Even when the frequency is provided in CPUID by the hypervisor, QEMU at least
requires a fairly explicit opt-in to advertise CONSTANT_TSC, presumably to try
to prevent users from shooting themselves in the foot.
