Re: [PATCH v5 22/51] x86/kvm: Mark TSC as reliable when it's constant and nonstop

On Wed, Jul 01, 2026, [email protected] wrote:
> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > index 909d3e5e5bcd5..1cef54e1e7d9e 100644
> > --- a/arch/x86/kernel/kvm.c
> > +++ b/arch/x86/kernel/kvm.c
> [ ... ]
> > @@ -1040,7 +1041,16 @@ static void __init kvm_init_platform(void)
> >  		}
> >  	}
> >  
> > -	kvmclock_init();
> > +        /*
> > +         * If the TSC counts at a constant frequency across P/T states and in
> > +         * deep C-states, treat the TSC reliable, as guaranteed by KVM.
> > +         */
> > +	tsc_is_reliable = boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
> > +			  boot_cpu_has(X86_FEATURE_NONSTOP_TSC);
> > +	if (tsc_is_reliable)
> > +		setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
> 
> [Severity: High]
> Does setting X86_FEATURE_TSC_RELIABLE here bypass SMP cross-CPU TSC
> synchronization checks?

No, KVM_FEATURE_CLOCKSOURCE_STABLE_BIT is for kvmclock, not for TSC.  The
CONSTANT+NONSTOP combination is KVM's way of guaranteeing cross-CPU synchronization.

> Forcing X86_FEATURE_TSC_RELIABLE based only on CONSTANT_TSC and NONSTOP_TSC,
> without verifying if the hypervisor actually guarantees cross-CPU
> synchronization (e.g., via KVM_FEATURE_CLOCKSOURCE_STABLE_BIT), sets
> tsc_clocksource_reliable = 1.
> 
> This bypasses the synchronization check in check_tsc_sync_source():
> 
> arch/x86/kernel/tsc_sync.c:check_tsc_sync_source() {
>     ...
>     if (tsc_store_and_check_tsc_adjust(false) || tsc_clocksource_reliable)
>         return;
>     ...
> }
> 
> Could this cause time to go backwards for guests running on hosts with
> unsynchronized TSCs when threads migrate between vCPUs?