Re: [PATCH v3] KVM: x86: Add x2APIC "features" to control EOI broadcast suppression

On 2 Dec 2025, at 10:06 PM, Sean Christopherson seanjc@google.com wrote:

• if (cap->args[0] & KVM_X2APIC_ENABLE_SUPPRESS_EOI_BROADCAST)
• kvm->arch.suppress_eoi_broadcast = KVM_SUPPRESS_EOI_ENABLED;
• if (cap->args[0] & KVM_X2APIC_DISABLE_SUPPRESS_EOI_BROADCAST)
• kvm->arch.suppress_eoi_broadcast = KVM_SUPPRESS_EOI_DISABLED;

Should this also update existing vCPUs by calling kvm_apic_set_version() for all already-created vCPUs? or is the intent that userspace must set these flags before any vCPUs are created? If so, should this warn if called after vcpus are created?

QEMU currently creates vCPUs before initializing the I/O APIC, so deciding KVM’s SEOIB behavior at vCPU initialization time is just a bit awkward. It’s manageable either way, I just want to confirm the uAPI semantics.

For reference, the initial QEMU side patch: https://patchew.org/QEMU/20251126093742.2110483-1-khushit.shah@nutanix.com/