From: Venkatesh Srinivas Date: Wed, 15 Jul 2026 23:42:35 +0000 (+0000) Subject: KVM: x86: Only reset TSC Deadline Timer in apic_timer_expired on KVM_RUN X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=e800decd9c0ac4349bcd8f8f9b29fd21fe93165e;p=thirdparty%2Fkernel%2Flinux.git KVM: x86: Only reset TSC Deadline Timer in apic_timer_expired on KVM_RUN On Intel platforms with a VMX preemption timer and APICv, if a VMM calls KVM_GET_LAPIC before KVM_GET_MSRS to save the vCPU state, it is possible to lose a pending timer interrupt. If the thread running these ioctls is migrated to another core after calling KVM_GET_LAPIC but before KVM_GET_MSRS and the guest is using their LAPIC timer in TSC-deadline mode, not only does the save LAPIC state not carry the pending interrupt, the TSCDEADLINE MSR will be zeroed. After migration across CPUs, KVM_GET_MSRS calls vcpu_load, posting the interrupt and clearing the MSR: vcpu_load() -> kvm_arch_vcpu_load() -> kvm_lapic_restart_hv_timer() -> start_hv_timer() -> apic_timer_expired() -> kvm_apic_inject_pending_timer_irqs() . post interrupt into the LAPIC state . clear IA32_TSCDEADLINE The saved LAPIC state will be missing the pending interrupt and the saved MSR will be zero. Oops. Fix by only posting an interrupt when we're attempting to enter the guest (vcpu->wants_to_run == true), not for vcpu_load from other paths. Assisted-by: gemini:gemini-3.1-pro-preview Debugged-by: David Matlack Debugged-by: Sean Christopherson Debugged-by: Jim Mattson Debugged-by: James Houghton Signed-off-by: Venkatesh Srinivas Message-ID: <20260715234234.15382-2-venkateshs@chromium.org> Reviewed-by: James Houghton Reviewed-by: Chao Gao Cc: stable@vger.kernel.org Fixes: ae95f566b3d2 ("KVM: X86: TSCDEADLINE MSR emulation fastpath", 2020-05-15) Signed-off-by: Paolo Bonzini --- diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c index 38bba9a1114c..48b019114c19 100644 --- a/arch/x86/kvm/lapic.c +++ b/arch/x86/kvm/lapic.c @@ -2052,7 +2052,7 @@ static void apic_timer_expired(struct kvm_lapic *apic, bool from_timer_fn) if (apic_lvtt_tscdeadline(apic) || ktimer->hv_timer_in_use) ktimer->expired_tscdeadline = ktimer->tscdeadline; - if (!from_timer_fn && apic->apicv_active) { + if (!from_timer_fn && apic->apicv_active && vcpu->wants_to_run) { WARN_ON(kvm_get_running_vcpu() != vcpu); kvm_apic_inject_pending_timer_irqs(apic); return;