On Wed, 2026-05-20 at 14:33 -0700, Sean Christopherson wrote:
> On Wed, May 20, 2026, David Woodhouse wrote:
> > On Wed, 2026-05-20 at 13:44 -0700, Sean Christopherson wrote:
> > >
> > > + /*
> > > + * If the TSC counts at a constant frequency across P/T states,
> > > counts
> > > + * in deep C-states, and the TSC hasn't been marked unstable,
> > > treat the
> > > + * TSC reliable, as guaranteed by KVM. Note, the TSC unstable
> > > check
> > > + * exists purely to honor the TSC being marked unstable via
> > > command
> > > + * line, any runtime detection of an unstable will happen after
> > > this.
> > > + */
> > > + if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
> > > + boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
> > > + !check_tsc_unstable())
> > {
> > > + tsc_properties = TSC_FREQ_KNOWN_AND_RELIABLE;
> >
> > kvmclock = 0; /* Why use it if the TSC works? The kvmclock exists
> > *purely* to work around a TSC which *doesn't*
> > have those properties checked above. */
>
> kvmclock still provides SYSTEM_TIME and WALL_CLOCK :-/
Um, SYSTEM_TIME *is* the kvmclock of which we speak, isn't it?
WALL_CLOCK is something else, and I guess we should still consume that,
yes.
>
> > > +
> > > + kvm_tsc_khz_cpuid = kvm_para_tsc_khz();
> > > +
> > > + /*
> > > + * If provided, use the TSC (and APIC bus) frequency provided in
> > > KVM's
> > > + * PV CPUID leaf even if kvmclock itself is disabled via command
> > > line.
> > > + * The PV CPUID information isn't dependent on kvmclock in any
> > > way, and
> > > + * in fact using the precise information is *more* important when
> > > the
> > > + * user has explicitly disabled kvmclock to force the kernel to
> > > use the
> > > + * TSC as its clocksource.
> > > + */
> > > + if (!kvmclock) {
> > > + if (kvm_tsc_khz_cpuid)
> > > + tsc_register_calibration_routines(kvm_get_tsc_khz,
> > > + kvm_get_cpu_khz,
> > > + tsc_properties);
> > > + return;
> > > + }
> > > +
> >
> >
> > Regardless of the above, why not just register these here
> > unconditionally, and remove the later call that does the same?
>
> Because if kvmclock=n, it's only safe to call kvm_get_tsc_khz() if
> kvm_tsc_khz_cpuid
> is non-zero, other wise the "else" path will hit a NULL pointer deref when
> trying
> to get the frequency from the PV clock struct:
>
> return kvm_tsc_khz_cpuid ? : pvclock_tsc_khz(this_cpu_pvti());
Ah, right. Thanks. Ick though :)
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