Re: [PATCH 1/1] KVM: x86: lapic: update apic_base and APIC ID together

[Chao Gao <chao.gao@intel.com>]

On Tue, Aug 18, 2026 at 04:42:38PM -0700, Dongli Zhang wrote:
>The commit b2849bec936b ("KVM: VMX: Update SVI during runtime APICv
>activation") resolved the loss of EOI issue when apicv is activated after
>being inhibited at runtime. However, it does not resolve the cause of the
>runtime apicv inhibition.

Why lead with commit b2849bec936b? APICv is inhibited and re-activated for
other reasons regardless of the race below, so that fix is needed anyway. The
race is at best an orthogonal issue.

>
>The inhibition occurs because apic_base and the APIC ID are not updated
>together.
>
>Although commit 052c3b99cbc8 ("KVM: x86: Reinitialize xAPIC ID when
>userspace forces x2APIC => xAPIC") reinitializes the xAPIC ID to the
>vCPU ID when userspace forces the APIC to transition directly from x2APIC
>to xAPIC mode, the updates are not performed in a single transaction.
>
>If another thread calls kvm_recalculate_apic_map() during the window
>between updating apic_base and the APIC ID, kvm_recalculate_phys_map() may
>set xapic_id_mismatch and temporarily inhibit APICv.

So the goal is to avoid a _transient_ APICv inhibit in a corner case?

If so, please spell out in the changelog why it's worth fixing.