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

On Wed, Jul 01, 2026, sashiko-bot@kernel.org wrote:

quoted

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

[ ... ]

quoted

@@ -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?