FAILED: patch "[PATCH] KVM: x86: Only reset TSC Deadline Timer in apic_timer_expired" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x e800decd9c0ac4349bcd8f8f9b29fd21fe93165e
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026072512-regular-reference-8cac@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From e800decd9c0ac4349bcd8f8f9b29fd21fe93165e Mon Sep 17 00:00:00 2001
From: Venkatesh Srinivas <[email protected]>
Date: Wed, 15 Jul 2026 23:42:35 +0000
Subject: [PATCH] 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.

Message-ID: <[email protected]>

index 38bba9a1114c..48b019114c19 100644
@@ -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;