Re: [PATCH v4 3/3] x86/tlb: add architecture-specific TLB IPI optimization support

[Sean Christopherson]

On Mon, Feb 02, 2026, Lance Yang wrote:
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 37dc8465e0f5..6a5e47ee4eb6 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -856,6 +856,12 @@ static void __init kvm_guest_init(void)
>  #ifdef CONFIG_SMP
>       if (pv_tlb_flush_supported()) {
>               pv_ops.mmu.flush_tlb_multi = kvm_flush_tlb_multi;
> +             /*
> +              * KVM's flush implementation calls native_flush_tlb_multi(),
> +              * which sends real IPIs when INVLPGB is not available.

Not on all (virtual) CPUs.  The entire point of KVM's PV TLB flush is to elide
the IPIs.  If a vCPU was scheduled out by the host, the guest sets a flag and
relies on the host to flush the TLB on behalf of the guest prior to the next
VM-Enter.

        for_each_cpu(cpu, flushmask) {
                /*
                 * The local vCPU is never preempted, so we do not explicitly
                 * skip check for local vCPU - it will never be cleared from
                 * flushmask.
                 */
                src = &per_cpu(steal_time, cpu);
                state = READ_ONCE(src->preempted);
                if ((state & KVM_VCPU_PREEMPTED)) {
                        if (try_cmpxchg(&src->preempted, &state,
                                        state | KVM_VCPU_FLUSH_TLB))
                                __cpumask_clear_cpu(cpu, flushmask);  <=== 
removes CPU from the IPI set
                }
        }

        native_flush_tlb_multi(flushmask, info);

> +             if (!cpu_feature_enabled(X86_FEATURE_INVLPGB))
> +                     pv_ops.mmu.flush_tlb_multi_implies_ipi_broadcast = true;
>               pr_info("KVM setup pv remote TLB flush\n");
>       }